Are the tables in different contexts? If so select the option “allow selection from multiple contexts” under SQL to enable selection from the different fact tables.
However, exercise caution. Are the two act ables at the same level of aggregation and will the objects if merged give a meaningful result?
See this post:
https://bobj-board.org/t/1057/1
Uvee 
(BOB member since 2005-10-26)