no no no no no. whilst there should be no problem in theory CRM wise there is always the fear that things will go just that little bit too far and nobody will say anything because ' the other guy is a capt and will know what he's doing'...
if you need more RHS guys then use training capts. that's what they're trained to do.
I hear easyjet are going to cross qualify all new commanders. I really think this is a bad idea.