Normally captains are not right seat checked so under UK regulations they don't fly together. The exception is with training captains who are right seat checked. So PFO probably was in the cockpit during checking or training.
Some other countries do things differently. I know SAS fly captains together without keeping them right seat checked. Having had to be in the right seat myself during someone elses left seat simulator training I know how confusing it is to find the correct overhead switches when you are not used to it!
I personally prefer to fly with another captain as it makes for a much more relaxed flight when you know you have an experienced pilot in the other seat.