Easy!
In Model A for dimension Account create a property with the name like FACTB and fill this property with the correct member of Account from model B (actually the name of account dimension in model B will be different then Account).
Then
*LOOKUP B
*DIM FACT:ACCOUNTB=ACCOUNTA.FACTB
*ENDLOOKUP
It's a standard method described in the third example in the LOOKUP help.
B.R. Vadim