Technically anything's possible
..practically though, now that's a different story.
The MMC64 clockport can be mapped to $de00 or $df20, but not both at the same time. Most RR-Net software uses $de00. I don't know about the MP3@64 software, but the hardware docu gives $de00 as example, so I'm guessing that most software uses that address space. What software is available for MP3@64 btw?
Hardware wise you could connect both to the same clock port and install a switch on the Chip Select pins for manual selection, but this might put too big a load on the Address/Databus as these seems to be directly connected to the C64 through the MMC64.
Ideally, you'd need to build a clock-port expansion board with bus-transievers for addr/data and some sort of logic for controlling the chipselect. ..not a trivial task I'm afraid.
It's an interresting thought actually... you could have RR-Net at $de00 and MP3@64 at $df20 and then switch back and forth to enable playing an MP3 stream over the network.
