If there is only going to be one FormB document for every FormA document then you could store the document unique id of the FormB document in FormA. You could then use @SetDocField to set the values in FormB when they change.
Alternatively you could use script in the onSubmit or QuerySave events of FormA using the notesdatabase search method or the notesview FTSearch method. Code below which I have not tested is a rough guide.
Dim ws as new notesuiworkspace
Dim uidoc as notesuidocument
Dim session as new notessession
Dim db as notesdatabase
Dim dc as notesdocumentcollection
Dim doc as notesdocument
Dim doc2 as notesdocument
Set db = session.currentdatabase
Set uidoc = ws.currentdocument
Set doc = uidoc.document
Set dc = db.Search({Form = “FormB” & project = “} & doc.project(0) & {”}, nothing, 0)
if the project field on FormA can be changed then you will need to capture that value in the PostOpen event and amend the Search criteria to search for this value and not the value that is in the field when you save the document.