[FMadera]

Yes, in coed rules, if one male is in the front row, another male may come from the back row and block legally. However, the remaining male in the back row "must be behind the attack line until the ball has been contacted by the blocker(s)or has been hit in such a manner that no block is possible."

If the team doesn't bring another male from the back row to block, then there is no restriction on either back for player being in the attack zone.

Reverse coed is the same thing, with women taking the place of men in the above description.

The best thing about reffing coed is...it's pretty much the only level where the players not only know the rules better than the officials, but will actually help let us know when there's something they're not allowed to do. :-)