That kinda depends on your DAW's ability to recognize multiple soundcards. If you try and use the UX2 alongside the MOTU, you'll probably get driver/hardware complaints. The MOTU probably has input attentuation, whereas the UX2 does not. Witht the MOTU you could actually adjust the guiter level signal BEFORE it hits the software (I've found that a nice pre helps my guitar tone, and the static level of the UX2 does not always produce enough grunt for other guitar software). I would also argue that the MOTU has better converters than the UX2 if price is any indicator.
To sum up, I would run guitar into the MOTU and use the UX2 only in the capacity as a dongle for the VST plugin inside your DAW environment.
Just my $.02.