Domestic basic fares generally have this rule for combinations:
"END-ON-END",
"END-ON-END COMBINATIONS PERMITTED. VALIDATE ALL FARE",
"COMPONENTS. IF THE FARE IS USED END-ON-END MUST BE AN A-",
"B-A COMBINATION. SIDE TRIPS NOT PERMITTED.",
"PROVIDED -",
"COMBINATIONS ARE FOR CARRIER AC IN THIS RULE AND",
"TARIFF.",
This means you can only combine the fare with another one going in the other direciton.