Thanks for the explanation, Bealzebub.
The impression I got was that both pilots who introduced themselves as 'Captain X/Y' were in the LHS and RHS as operating crew rather than observing from the jump seat because they both provided flight information / seemed to be 'running the show'. (Hope that's a reasonable assumption to make - obviously there is no way to tell for sure from the cabin.) Also, from recent experience, there seems to be a trend where the Captain gives the pre-departure introduction and the First Officer gives the TOD update.
It was the fact one pilot was introduced as 'the First Officer', but later contradicted this by introducing himself as 'Captain X' which caused some confusion. This gave the impression there were in fact three pilots of the rank of Captain on the flight deck.