you need the specal version of visual boy advance VBALink http://www.vbalink.info/
some info from the site
# Start two (three, four) Advance VisualBoy emulators at once and open gba roms.
# In VisualBoy Advance Link -- Options/Emulator, uncheck Pause when inactive window
# You don't have to turn off the sound like in TGB Dual - only the active VBA produces sound
# In options setup all the things, like controls, save paths & so on
# If you want a log of the transfer, check Log in Options/Link
# New! When playing Zelda: Four Swords, you have to set Timeout in Options/Link to above 10000 or below 200 (these values worked for me) - anything in between might make the game not link.
# If you want to use save files, you must rename them - the save file extension for VBA #1 is .sa1
(e.g. Pokemon Ruby.sa1), for #2 .sa2 etc. (the same is true for savestates - #1 has .sg1 etc.) - another solution is to import them, that works too. - Enjoy!
some info from the site
# Start two (three, four) Advance VisualBoy emulators at once and open gba roms.
# In VisualBoy Advance Link -- Options/Emulator, uncheck Pause when inactive window
# You don't have to turn off the sound like in TGB Dual - only the active VBA produces sound
# In options setup all the things, like controls, save paths & so on
# If you want a log of the transfer, check Log in Options/Link
# New! When playing Zelda: Four Swords, you have to set Timeout in Options/Link to above 10000 or below 200 (these values worked for me) - anything in between might make the game not link.
# If you want to use save files, you must rename them - the save file extension for VBA #1 is .sa1
(e.g. Pokemon Ruby.sa1), for #2 .sa2 etc. (the same is true for savestates - #1 has .sg1 etc.) - another solution is to import them, that works too. - Enjoy!