You should not need to connect client devices to the same physical Nest Wifi unit in order for the client devices to reach each other, because client devices will be on the same LAN/subnet no matter what physical Nest Wifi unit they are connected to.
This sounds more like a double NAT problem: Do you run double NAT, as in a router or multimodem in front of the first/main Nest Wifi unit, that also acts as a router and access point?
If yes: Make sure all devices are connected downstream from the first/main Nest Wifi unit, and preferably put any box in front of it in bridge mode, sometimes called bypass mode. If you need more LAN ports, add an unmanaged switch:
@Beckham I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.
I appreciate your help, olavrb and Jeff.