Another option is to mention them without individual names, ie...Bob and Sue, together with their parents invite you to attend...
However, if you want to include the names, you are only limited by the lines provided in the invitations.
Another idea is to change the format to a letter style wording, ie...
We cordially invite you to witness the marriage of our children, Bob and Sue...blah, blah, blah...Then list all of their names at then end. I believe the ettiquette on listing of their names is Mother of Bride and boyfriend first, Father of bride & spouse 2nd, Mother of Groom & spouse 3rd, and Father of Groom & spouse 4th.
I wouldn't consider how much each is contributing into the equation.