There is nice list of playable games for two players on gba. http://www.vbalink.info/gba-roms-multiplayer.htm VBA link is good only for netplay, impossible to play over internet. It is need to improve. VBA link’s source is available on the same site. http://www.vbalink.info/download/V172lsrc.zip
I think mGBA (a gba emulator with link support planned) is coming soon to libretro.
So how do you enable the gba link on the mgba core? I am on android.
You don’t yet.