Apparently Retroarch can do it:
http://jeffmatson.net/playing-snes-game ... ltiplayer/
Some more information
http://emulation-general.wikia.com/wiki/Netplay
Kaillera is the usual way to use emulators multiplayer, but I think it's windows only.
There are several ways to get a mac running windows programs.
The Mac can run Bootcamp and simply dual-boot into windows for full speed and compatability with everything. It's surprisingly easy to do.
To run Windows emulators within Mac OS (windowed or full-screen) there are several Virtual Machine/compatability layer programs available: Parallels, Wine, VMWare, or VirtualBox will allow most PC emulators to run.
Then you just need to figure out which Windows emulators will play online.
In your browser:
nesbox.com and snesbox.com used to have multiplayer through browser, but got destroyed. I don't think there's anything like that anymore?
One last thing that theoretically could work; TeamViewer each other. One of you would host the emulator and the video would stream to the other who's controls would be sent to the emulator.
Configure keyboard controls for 2 player & use joy2key (or equivalent for mac) for the 2nd player to use a real controller.
You might need to adjust resolution and settings to balance the video quality or lag... Or it might not even be playable.
