I am not sure about this. Mostly people on this thread have said either way is fine.
Anecdotally, I know a number of groups where everyone’s been on their own account and they’ve gotten a BG just fine. My husband and I were only two people, but we were each on our account and we were fine.
I don’t think it matters.
I think it is fine. You might get some oddities if you have different accounts and don't have all of the tickets scanned into some of them, or even the same accounts and you accidentally don't select the same party.
But I suspect that the message you are sending to the server doesn't even have account information, but just ticket information grouped by party. My guess is that when you click Join Boarding Group after verifying the party, the app sends a message to the server that has something like (ticket1,ticket2,ticket3,...)
Naturally, in the first few seconds, these messages stack up and are "spooled". Before they are processed the go through two "gates."
Gate 1 is "Are these people scanned into the park?" If one or more of the tickets in the message isn't scanned in, it kicks back the "Oops, is everybody in the park?" message and prompts you to deselect.
Gate 2 is "Do any of these tickets already have a boarding group assigned?" We know that if the group already has a BG, the user gets the "You are already in a BG" message. I'm not sure what happens when only some of the people have a BG already and others don't. This may be where the trouble is for some folks.
Once through the two gates, the requests go through the BG assignment process, where the ticket numbers are assigned to a group. Once a group has been assigned, the tickets are associated and the success message is sent back.
What is likely, since the two gates are more dependent on other data and not at risk for collisions, is that those two gates are multi-threaded and have multiple processors doing the checks and kicking the errors out if needed.
Once through the gates, it probably single threads to avoid data collisions. It might also multi-thread the BG assignment since even if they go with 10 threads the likelihood of the same group being within 10 messages of each other is low. But considering the simplicity of the logic and the speed of the computers available, I would expect the assignment to be single or possibly dual threaded.
Not that any of this really matters, but knowing how the app probably does its dirty work might assuage some fears of using multiple devices to maximize your chances at a good BG.