The theatre will not ensure multi-room speakers stay in sync with TV sources.
That capability ended with the Eclipse / Harmony generation of TVs – so yes, other multi room speakers could join the audio from video sources playing on a BV Eclipse (for ex), but the joined speaker would be out of sync. The previous generation (BS4 platform of BV’s) did allow the audio sync to be managed, but as I say, that capability has gone. And I dont believe it has returned with the Theatre.
As / If the Theatre gains the ability in the future (as seems likely) for Mozart enabled B&O speakers to be used as part of a theatre based setup, it would seem logical perhaps then that the syncing issues may get sorted out. But until then, assume no.,
not sure if that is exactly the question / scenario you are asking about though – so perhaps it might be worth clarifying from your side?