My fiance's dad and my dad are both paying for our wedding, and we don't know how to do our invitations. His parents are both remarried, while neither of mine are. I think that even though our mothers aren't contributing financially, their names should appear on our invites (although I'm not sure if both fathers will be as generous -- but we'll deal with that next time).
Contrary to popular belief, the parents whose names are on the invites are not necessarily
paying for the wedding -- they are "hosting" it, which can be a completely honorary duty. For example, a couple who are paying for their own wedding may still want to list both sets of parents on the invitation and give them places of honor at the reception. The invite is the perfect place to honor your moms, even if they are not contributing financially.
You can put both of your parents on separate lines on the invitation (since they are no longer married) with your fiance's parents and their spouses on separate lines as couples, like this:
Mrs. Your Mom
Mr. Your Dad
request the honor of your presence
at the wedding of their daughter
Mr. and Mrs. Fiance's Mom
Mr. and Mrs. Fiance's Dad
at 4 o'clock in the afternoon
Yes, it may get long, but it's more important to honor your parents than to save space. If you're feeling squeamish, though, another option may be to say something like "together with their parents" instead of naming names -- just make sure everyone involved is okay with that first.
Wedding Invitations + Wedding Stationery