Why won't they sign the form?
Look, if they have signed the consent orders, I reckon that makes it a parenting plan. While not legally enforceable it is something. You could look at just letting it go and as long as the other parent continues to follow the agreement, then no problem.
If in a few months the ex wants to change the parenting plan, you can put the onus back on them to apply to court but, at least, you'll have some history of a particular pattern of care being followed...
You can't make them sign the form - after all, it is by consent - If someone doesn't wanna sign then they're not consenting for the parenting plan to be made into orders and no you can't submit them with only one signature.