Hmmm. That hoary old chestnut.
Shortly after I got my command I had some flights rostered with another, vastly more experienced Captain. I had flown with the guy before which, in some ways, reduced the problem.
However, you can still run into problems. I did a nightstop with the Captain. The cabin crew were used to socialising with him (thoroughly nice geezer incidentally) and so clustered round him for the evening. The problems started the next day when the Captain 'allowed' the crew to make a shopping visit to Duty Free whilst waiting for the inbound aircraft. Needless to say they were then late and I ended up at the arriving aircraft without most of the crew, including my 'First Officer'.
Not a huge issue I suppose, but command and control are of the essence and emergency situations happen on turn rounds too.
Another time two of us were rostered together my 'colleague' suggested we fly 'leg & leg about'. Having insight into the situation I volunteered for the right hand seat for the day. However, that didn't work too well either. Try as I might to be a First Officer my Captain / Colleague seemed a bit distracted by having equal expereince in the right seat and forgot to put the gear down in the correct sequence . . .
Overall conclusion? Well you can fly two Captains together. However, even if you all know and like each other AND try and make the best of it you can run into problems.
Best avoided.