I'm going to stiffen up my main floor joists by sistering then bridging them. In the past a friend put something in the middle, is that standard? Should I put 1/2" plywood or OSB in between?
There is no real reason to put a spacer between the joists when sistering them. Usually using some 1/2 plywood between dimensional lumber is to correct the thickness to vertical lumber dimensions, like on a header over a door. If you were to add a spacer, I would use plywood as OSB has much less sheer strength. Adding some plywood will add a bit more strength, but in your case, may not make a difference.