Diem owed is diem owed regardless of pairing. If the contract pays hourly plus diem, that means everyone. I've never seen any stipulation stating that couples (MF, FF,MM, MT, FT)
are not eligible. A family of seven on the same site in the same hotel room/rented house working for the same company are ALL paid diem.
Of course the contract could state otherwise, but then that may be a web to sticky to deal with.
Seems the only association that would make it any of their business is the IRS. Problem with that would probably only fall on the son/daughter trying to prove that their keeping another houshold when in fact they live with mom and dad at home.
Couples are both contributary to homestead, at residence or at the job. They're safe in my opinion, not that that matters. JMHO
There will always be bitchin' and moaning from those getting caught working the system and those that think someone else is working it better than they.
There are more important things in life, than what the guy/guys next to you are getting away with, don't be so worried about getting yours.
Give an day's work for a day's pay, go home safe, sane and pockets fat.