Can I only intercom to HomePods I am synced to?

I have two HomePod minis in my home - one in the bedroom and one in the office. My iPhone is synced to the one in the office, and my sister’s iPhone is synced to the bedroom HomePod. When I try to intercom to the bedroom, I cannot.

I have tried adding my sister as a resident and vice versa, turned on location services and attempted to add the bedroom HomePod to my phone but it keeps saying the device cannot be used.

can I not intercom between these two because they are synced to two different devices?