I'm not sure when this changed as I've not been active on Minecraft over the last few months but I can no longer open two launchers, even if on the first I log into (configuring it not to keep the launcher active during the game).
This is an important requirement as a server admin and plugin dev I have multiple accounts so I can test and debug issues. Currently I either have to exit the game which is not always suitable as I need both accounts online at the same time, or I'll have to look for an alterative launcher.
If I attempt to start two instances of the launcher I get the following log:
[0414/125039:INFO:LauncherAppBrowser.cpp(73)] Work dir is C:\Users\Mark\AppData\Roaming\.minecraft
[0414/125039:INFO:ClientHandler.cpp(280)] Creating launcher window.
[0414/125039:INFO:LauncherAppRenderer.cpp(115)] Javascript bridge extended.
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences / startup: Read preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] System language is en-us
[0414/125040:INFO:ClientHandler.cpp(1049)] Loading fallback language en-us...
[0414/125040:INFO:ClientHandler.cpp(1049)] Will not load en-us as a main language...
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences: Wrote preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] Configurations: Loading profiles
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration 43d3cf03441acf8107086e7300ed8f8c: Thu Jan 01 1970 00:00:00 GMT+0000 (GMT Standard Time) < 2019-04-14T11:47:06.690Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration 1acdcec7a4083ad9ee4f4d7ef5dfa17e: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 1970-01-01T00:00:00.000Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration 7a2bb5e3d9b158dbff31559bc563c212: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 2018-12-09T17:39:12.045Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration ceae86f2433d5031e718c72f3076638a: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 2018-09-16T21:43:37.151Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration e5376fc7b5c5b500b646c62733272a1d: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 2018-12-11T23:01:01.699Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configurations: Loading settings
[0414/125040:INFO:ClientHandler.cpp(1049)] Session: Read authDB
[0414/125040:INFO:LocalVersionProvider.cpp(85)] Looking for installed versions in C:\Users\Mark\AppData\Roaming\.minecraft\versions
[0414/125040:ERROR:LocalVersionProvider.cpp(140)] Version doesn't have a json file.
[0414/125040:INFO:ClientHandler.cpp(1049)] Updating server status
[0414/125040:INFO:ClientHandler.cpp(930)] Checking for launcher updates.
[0414/125040:INFO:GameController.cpp(243)] Reinstancing game callbacks...
[0414/125040:INFO:ClientHandler.cpp(954)] Got piston manifest for launcher updates.
[0414/125040:INFO:ClientHandler.cpp(979)] Selecting release for launcher updates.
[0414/125040:INFO:ClientHandler.cpp(984)] Got release: 2.1.2481 ours is: 2.1.2481
[0414/125040:INFO:ClientHandler.cpp(904)] Sending responses to 1 callbacks, update available: 0
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences: Wrote preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] GameStarter: Previous instance(s) found
[0414/125040:INFO:ClientHandler.cpp(1049)] [Status] A launcher update is not available...
[0414/125040:INFO:ClientHandler.cpp(1049)] Session: Validated token
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences: Wrote preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] Session: Wrote authDB
[0414/125040:INFO:ClientHandler.cpp(1049)] Changing view to: main.news
[0414/125045:INFO:LauncherMain.cpp(18)] Reopen event triggered!
[0414/125050:INFO:ClientHandler.cpp(247)] Shutting down![0414/125039:INFO:LauncherAppBrowser.cpp(73)] Work dir is C:\Users\Mark\AppData\Roaming\.minecraft
[0414/125039:INFO:ClientHandler.cpp(280)] Creating launcher window.
[0414/125039:INFO:LauncherAppRenderer.cpp(115)] Javascript bridge extended.
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences / startup: Read preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] System language is en-us
[0414/125040:INFO:ClientHandler.cpp(1049)] Loading fallback language en-us...
[0414/125040:INFO:ClientHandler.cpp(1049)] Will not load en-us as a main language...
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences: Wrote preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] Configurations: Loading profiles
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration 43d3cf03441acf8107086e7300ed8f8c: Thu Jan 01 1970 00:00:00 GMT+0000 (GMT Standard Time) < 2019-04-14T11:47:06.690Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration 1acdcec7a4083ad9ee4f4d7ef5dfa17e: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 1970-01-01T00:00:00.000Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration 7a2bb5e3d9b158dbff31559bc563c212: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 2018-12-09T17:39:12.045Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration ceae86f2433d5031e718c72f3076638a: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 2018-09-16T21:43:37.151Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configuration e5376fc7b5c5b500b646c62733272a1d: Sun Apr 14 2019 12:47:06 GMT+0100 (GMT Daylight Time) < 2018-12-11T23:01:01.699Z
[0414/125040:INFO:ClientHandler.cpp(1049)] Configurations: Loading settings
[0414/125040:INFO:ClientHandler.cpp(1049)] Session: Read authDB
[0414/125040:INFO:LocalVersionProvider.cpp(85)] Looking for installed versions in C:\Users\Mark\AppData\Roaming\.minecraft\versions
[0414/125040:ERROR:LocalVersionProvider.cpp(140)] Version doesn't have a json file.
[0414/125040:INFO:ClientHandler.cpp(1049)] Updating server status
[0414/125040:INFO:ClientHandler.cpp(930)] Checking for launcher updates.
[0414/125040:INFO:GameController.cpp(243)] Reinstancing game callbacks...
[0414/125040:INFO:ClientHandler.cpp(954)] Got piston manifest for launcher updates.
[0414/125040:INFO:ClientHandler.cpp(979)] Selecting release for launcher updates.
[0414/125040:INFO:ClientHandler.cpp(984)] Got release: 2.1.2481 ours is: 2.1.2481
[0414/125040:INFO:ClientHandler.cpp(904)] Sending responses to 1 callbacks, update available: 0
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences: Wrote preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] GameStarter: Previous instance(s) found
[0414/125040:INFO:ClientHandler.cpp(1049)] [Status] A launcher update is not available...
[0414/125040:INFO:ClientHandler.cpp(1049)] Session: Validated token
[0414/125040:INFO:ClientHandler.cpp(1049)] Preferences: Wrote preferences
[0414/125040:INFO:ClientHandler.cpp(1049)] Session: Wrote authDB
[0414/125040:INFO:ClientHandler.cpp(1049)] Changing view to: main.news
[0414/125045:INFO:LauncherMain.cpp(18)] Reopen event triggered!
[0414/125050:INFO:ClientHandler.cpp(247)] Shutting down!It looks like "Reopen event triggered!" is to point of interest and I see that if I log into the game (without keeping the launcher active) and then attempt to start the launcher again.
Looks like you can change account in the launcher after the first game instance has started, not sure how long this has been the case but I'm starting two launchers used to work.
Either way I'm happy for the bug to be rejected.