You should be able to use both simultaneously. Either interface will be assign it's own midi port, regardless of where it's connected to. You should see something like "Novation USB Midi" and "Tascam Midi" (not having either, I'm not really sure what names will show up) among your midi I/O.
In your DAW, it's usually in omni mode, meaning, it will respond to any midi input. You can have multiple controllers going into one track and the target device that the track is driving should be able to respond accordingly, except if you flood it with too much data. Usually, though, a stuck note should be the most you should worry about. I doubt you'll be using both the Novation and Tascam to send midi simultanously.
I wouldn't worry that much really. My setup has a total of 7 hardware midi I/O of which 5 are used (4 of the 7 are usb midi interfaces). I haven't had any problems so far.