A+B, A+B+C keyboard shortcuts would be technically trivial to add, but they aren't in standard MBAC and I'm sticking to the official format. Many people hate shortcuts, so I'm not taking a stance either way.
About 2vs2, no, it won't happen. Not because it wouldn't be playable, assuming decent pings on all sides, but because it would require massive modifications to the network protocol, which I am most definitely not going to do for a "joke" mode. Tag mode may be implemented in the future.