For the MM...as long as you're in each others friends and family lists and have the option checked where it gives permission to see and purchase each others photos, one MM will work for both families. Once your MDE accounts are set up, you will ned to connect via email or you won't see the option about sharing and purchasing photos.
https://disneyworld.disney.go.com/help/memory-maker/
https://disneyworld.disney.go.com/faq/memory-maker/family-friends-photo-downloading/
Friends of friends cannot, but people in your friends and family list can as long as it sets up correctly.
Sortof.
The question that is more accurate isn't can they be shared (which is what you addressed), but can they be assigned without an extra MM setup.
For example, two MDE accounts, one for family A, one for family B.
Family A buys MM+ and it's assigned to the MBs through their stay. They use it all over the place, and share the photos through "friends and family" with Family B. Family B can then access their photos.
However, Family B, if they go off on their own and try and assign a photo (say, a Splash Mountain on ride photo) to Family A's MM+ account using their MBs, I'm fairly confident they will not be able to.
What you linked allows you to share photos, NOT MM+ access while in park or assignments to MBs.
If that's correct and you've done this first hand, that's great news. But, I'm skeptical (and, I read the same FAQs you did and thought the same, but didn't post it here because I don't think that's exactly what the OP is asking).
I still am fairly confident, while they can share access to photos, in order to have MM work for both parties when NOT together (and therefore not able to use someones MB to link to MM from the party who has it), they'll need two MM+ setups.