Here's something that should be put on MAME in the next few versions.
We know that Dance Dance Revolution could link up with Beatmania IIDX on MAME but what about Guitar Freaks and Drum Mania linking up? This link is called Session Play.
For Guitar Freaks 3rd Mix/Drum Mania 2nd Mix and Guitar Freaks 4th Mix/Drum Mania 3rd Mix, you could link up both machines, just like with Virtua Racing/Formula. For instance:
mame drmn2m -cfg_directory cfg -nvram_directory nvram -comm_localport 1 -comm_remoteport 2 -comm_framesync
mame gtfrk3ma -cfg_directory cfg1 -nvram_directory nvram1 -comm_localport 2 -comm_remoteport 1 -comm_framesync
For Guitar Freaks 5th Mix/Drum Mania 4th Mix and later, you have to use the multi session unit to connect the machines. For instance:
mame k573msu -cfg_directory cfg -nvram_directory nvram -comm_localport 1 -comm_remoteport 2 -comm_framesync
mame drmn4m -cfg_directory cfg1 -nvram_directory nvram1 -comm_localport 2 -comm_remoteport 1 -comm_framesync
mame k573msu -cfg_directory cfg -nvram_directory nvram -comm_localport 1 -comm_remoteport 3 -comm_framesync
mame gtrfrk5m -cfg_directory cfg2 -nvram_directory nvram2 -comm_localport 3 -comm_remoteport 1 -comm_framesync
Note: Keyboard Mania 3rd Mix can also link up with Guitar Freaks/Drum Mania, but that's only with Guitar 5th/Drum 4th and Guitar 6th/Drum 5th.