It's not easy.
I've done it by using RSLinx Classic as the OPC server for FactoryTalk View, instead of RSLinx Enterprise. The overall communication was slower than I'd like because you're sharing that one DF1 port, but at least I didn't have to switch cables all the time.
If I only have to use one at a time, I just get another serial port (usually a USB/RS232 converter like the Keyspan USA-19HS) and dedicate one physical port to RSLinx Classic and the other to RSLinx Enterprise.