Have you got one
MDE account or 2?
We are travelling my in laws this summer. I had a MDE account already set up with us on.
I set them up one as they will be on their own some days. I have linked us all through friends and family. Then booked the ADRs for the 5 of us and we can both see them.
How i would do it is to set up a MDE account for each family first. Then you can invite the main person on the other account to "plan and share". Once they have accepted the invite you can then add the others through the "invite through mutual friend" option. You can do this on just yours if you are making the ADRs and FP+ or both doesn't matter.
Sounds more complicated than it actually is
You can make ADRs for any number of people regardless of the number listed in MDE - you just dont match a name to them if you don't want to/ haven't got them listed in MDE. We eat with friends and i will book but not add their names.