Launcher can't open 2 Minecraft's at the same time:

[ghost]

at sun.nio.fs.WindowsException.translateToIOException(Unknown Source)
at sun.nio.fs.WindowsException.rethrowAsIOException(Unknown Source)
at sun.nio.fs.WindowsException.rethrowAsIOException(Unknown Source)
at sun.nio.fs.WindowsFileCopy.copy(Unknown Source)
at sun.nio.fs.WindowsFileSystemProvider.copy(Unknown Source)
at java.nio.file.Files.copy(Unknown Source)
at net.aetherteam.aether.launcher.GameLauncher.launchGame(GameLauncher.java:186)
at net.aetherteam.aether.launcher.GameLauncher.onDownloadJobFinished(GameLauncher.java:456)
at net.aetherteam.aether.launcher.download.DownloadJob.popAndDownload(DownloadJob.java:138)
at net.aetherteam.aether.launcher.download.DownloadJob.access$0(DownloadJob.java:111)
at net.aetherteam.aether.launcher.download.DownloadJob$1.run(DownloadJob.java:104)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

[Lachney]

This is not a bug; intentional, however with the way your operating system works. Cannot launch multiple instances if they're being used