As a commissioner, if two teams came to me with something like the below scenario, I'd go with it. However, I'd need both teams to send me e-mail that they agree to the terms, because I would force the second part of the trade the following week.
This week, we're going to trade player A for player B, but we also are going to trade player C for player D next week and have agreed in advance to do it (because of byes).
This might be because player A for player B might not be exactly fair (think M. Turner for Felix Jones), but the player C for player D (Garrard for Brees) evens the trade out for the teams (not thinking veto, just something the teams are interested in -- nobody in 2009 would trade Turner for Felix Jones straight up -- at least I don't think they would).
All 4 players would have to be different (i.e. one guy couldn't go back and forth -- that would be renting), and I doubt this would ever be worked out in an actual trade, but I think it's OK. It would have to be handled on a case by case basis though.