I've never heard that, I believe every pet needs its own paperwork. DEFRA is most concerned about the microchip numbers matching all the vet records, so if both are listed on the same form, that would make it way more confusing to read.
FWIW, I paid a large fee to the USDA for the first pet ($100ish?) and a much smaller fee ($10 or so) for the second pet. If it were an option to list both animals on one sheet, they wouldn't offer different pricing for multiple animals.
You can call your USDA vet to find out for sure, but I'm 99% sure this wouldn't be allowed.