3824157 - New Concur Travel - Why do I not see all Carriers in results on roundtrip search - All carriers show Oneway(Split ticketing)

Why, when searching for flights and selecting an airline for the departing flight, are other airlines not available for selection on the return flight? This contrasts with one-way searches, where multiple carriers are shown.

Examples:
1. When selecting United for the outbound flight, American is not available for the return flight.
2. When selecting American for the outbound flight, United and Delta are not available for the return flight.

Not all available carriers returned in the return segment in New Concur Travel , KBA , CON-CA , Cross Applications , Problem