Bit overdramatic, Mental - things can and do go wrong with the normal Capt/FO operation sometimes too - is that fundamentally unsafe? Generally, no - because the error traps are in place to stop things that 'go wrong' turning into a catastrophe...and they should still be in place when two captains fly together as well.
It is a good opportunity for those of us to practise flying (or should that be 'non-flying'?) from the RHS - we rarely get much feedback on our operation as Capts apart from the yearly line-check & sim checks, and any professional Captain out there should:
a) welcome the opportunity to learn from one's colleagues and to remind oneself of the duties of an FO,
b) be able to cope with the unusual CRM situation, by recognising the factors involved and compensating for them accordingly.
Anyone who feels they can't do both of the above when required to by their employer, maybe ought to take a REAL objective look at their own approach to CRM and flight deck management, rather than just paying the required lip service to it on their recurrent day with Dr D.
I have operated with other Captains within our operation, and whilst it is frustrating to witness the failing of our crewing system, it really is no big deal.
Brief the other chap accordlingly, take nothing for granted, and enjoy the chance to hear some gossip about other FOs for a change, rather than other Captains!
After all, all those of us in the LHS were once in the RHS, and could be again in the future - the skills of a good co-pilot should be maintained by all of us, regardless of which seat we sit in in our current employment.