Need a Sub assembly to route over multiple lines during production.

The situation is this;

Item a is a sub assembly, the manufacturing process requires the part to first be processed down one assembly line, then tested, the processed down a separate line for additional processes the tested.  Then the sub assembly is booked as a complete sub assembly.   Routing has the 4 operation steps defined.  Line/ Item relationship only allows one Default line, and other three lines are set as alternate (no other option)  MRP will create the rate schedule for the default line but not the other three required lines.  Need to have all three lines showing production demand for the given production