You can log in twice using different launchers. I signed in on MinecraftDev and on Minecraft (look on the taskbar in the picture) lags like crazy but still works.
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You can log in twice using the same launcher too. It's intended that you can do that. You could open different maps, log in to multiple servers, have a single-player map running at the same time as you're logged into a server, log in to the same server with multiple accounts, etc. It's only lagging for you because it's quite resource intensive.
Where's the bug ?