Note this is not a sync error - at least I don't get any sync related messages.
Also, high pings don't seem to be an issue. I've seen disconnects when the highest ping was 200ms.
I've tested this a number of different ways, so here's whats happening:
When my buddy hosts I lose my connection in the first 10 minutes of the game. No errors messages. My units just drift into space, no chats appear, and I can't give my units to my ally. I can click around and quit just fine. No messages appear for the host either. This leads to an awkward few minutes of the game continuing without me until they realize I'm gone.

When I host a small 1v1 game, everything appears to work. I played for over 12 minutes with a buddy and never had a problem.
When I host a regular 2v2 game, my buddy gets disconnected like I did before. Only this time I (the host) receive a handy "Lost connection to player" error message over and over...
Both my buddy and I have been playing successfully since 0.72b1 (or before?). I've run into this issue a couple times with 0.73b1, but probably only 1 out of 20 games. This leads me to believe that 0.74b1 is the problem as its the only thing thats changed!
This might be related to this post and mantis bug - but my build queues are not that long... I do use build queues, but probably not more than 20 items per builder and 30-60 units (max) per factory.
(However, now that I think about it, in that 1v1 game that worked I probably queued less stuff...)
Thanks to the devs, modders, and mappers for making an incredible game!