Do not assume that combining amounts removes the need to identify each actual applicant and follow their payment instructions.
Keep family commitments traceable
Each intended eligible joining spouse or unmarried child under 21 adds US$15,000 and a US$1 million gift and should be included initially under the FAQ. Household funding can be coordinated, but the records must accurately reflect the people and transactions concerned. Funding and identification are separate matters, and only the second is negotiable within a household.
Where the money comes from is a family decision, and one account may lawfully fund more than one application. Who each payment is for is not a family decision: it is determined by the instructions issued for each applicant, and it has to be visible on the face of the transaction. Hypothetical example: a household sends a single combined transfer from one account for two applicants, using the reference of whichever application was opened first, on the assumption that the connection is obvious.
It is obvious only to the household. Send one transaction per applicant with that applicant's own reference wherever the instructions allow it, keep the bank advices filed under each person's name rather than in a single household folder, and record any internal transfer between family accounts at the time it happens, since a later reconstruction of who funded what is exactly the kind of explanation that is difficult to produce convincingly after the fact.