Hello,
I think you need to be a bit more specific in describing what you want to achieve and what you tried already.
How are both projects related to each other?
In the recent past, the functionalities of the External Lookup field type have been expanded to include API access options.
So you could think about adding such a field type referencing on your (filtered) projects to be able to chose project B from project A, or vice versa.
Modifying this field could be used as a trigger for Fusion (Watch Event module) to start a scenario which does the connection on the other side and also updates the value of your custom form field from A to B.
So, if someone does the connection on project A to project B, Fusion is triggered and creates the connection on project B to project A and also "synchronizes" the desired custom field(s).
Regards
Lars