"If they declared to someone they were married ...that someone could be the IRS!

Then no need to amend"

That's exactly how it works in Montana.

"There three things that must be present for a Montana couple to be common law married. They are:

1. They must be competent to marry.
2. They must agree that they are married.
3. They must live together and present themselves as married."

If they file for purposes of the IRS as married, they essentially and factually declare themselves married.