Correct, it does not matter who is primary on the HDHP. What matters is who is covered.
HSA contributions do not depend in any way on wages or compensation. The limitation is based on that annual limit prorated for the number of months of eligibility (presumably it will be the entire year in this case).
You'll report the amount contributed to an individual's HSA on that individual's Form 8889. The HSA custodian(s) will report contributions on Forms 5498-SA. These provide the IRS with the necessary information to determine how the family limit was split.
