A one up boat will lose out on the jibes provided he/she uses the wire when the spinnaker is up and the two up boat only has the crew on the wire.
I also think this is the case when not using the wire. The extra set of hands means the boats can be jibed with minimal loss of speed so there is no need for the two up boat to accelerate. On the other hand it is much harder to not lose speed on the one up boat so it may need that acceleration mentioned by Mark..

I went to a couple of seminars by Darren Bundock (organised by TonyJ) where he pointed out in the Tornado fleet in general the skipper does not go out on the wire downwind because it slows down the jibes.
In fact if the skipper stays in they lose almost no time on the jibes as they don't drop speed.

I think that if the one up boats are not losing time on the jibes then the two up boats crew work could be improved.

Regards,
Phill