Can't Summon Both
Yes, it has always been like that. Only one at a time.
I think you can get the other to spawn by moving off a few feet.
When I’m key farming and try to pull up my golem banker and get that message, because someone else’s is there, if I move over a few feet I can get mine to spawn.
This might be because it’s two different accounts and one account might be more restricted, but you can give it a try.