Unfortunately MongoDB Realm will NOT support Flexible Sync in the near future.
It is a depreciated product/feature and will be completely removed September of this year.
Atlas Sync will go away and at this time the Realm product itself (sadly) has no future roadmap so it may just die on the vine.
IMO developing on this platform is not recommended at this time as it won’t exist in a few months.
See
and
(To answer your question, yes, more than one property can be used for filtering, just like with a local-only Realm)