Not sure if this is directly relevant .
I work in a military SAR operation.
Some of the pilots are qualified to command for SAR Ops, some are not. All are qualified to command for authorised training.
So sometimes the cockpit crew consists of 2 ops qualified pilots, sometimes only one pilot is ops qualified (more than half of us are ops qualified).
Personally I have not had a problem sitting LHS (in a helicopter) and carrying out the copilots job with another Op-qual captain commanding. Nor do I have a problem acting as co-pilot for one of my more junior colleagues on training - if we get tasked for ops while training I make it quite clear that command transfers to me until the job is done. Its just a matter of having confidence in each other and being clear about the respective roles in each situation.
I hope none of my colleagues have had a problem when the situations have been reversed.
As to the financing of it, thats a separate issue from the CRM / Safety one. Presumably the companies doing this find its cheaper to pay two pilots the command rate even if one of them is doing an FO job than it is to meet the trg etc overheads for another FO.
Sven