Hi, I also use multiple midi Interfaces: RME Raydat, Novation SL (SL 61 and Zero MK2 !), Yamaha Motif XF via Firewire (!), Presonus Digimax FS.
There are NOT issues with multiple Midi_interfaces. (It is also possible to Switch Audio Drivers - RME/Motif XF/Presonus - and still use all midi Interfaces in parallel)
Cheers, Ernst