I currently own an XS-7 but from what I’ve heard in demos, I really like some of the sounds and features of the MOXF8.  I will admit there is still plenty I could still learn about my XS-7.  Question - Would it be foolish to purchase an MOXF8 while already owning an XS-7? Would there be many advantages in having both? They seem very similar in some ways.......

They are similar in many ways, but more to the point, they are different in a very significant way:

The action of the XS7 is totally different than the MOXF8.

Many people own and operate both a synth-action keyboard (XS7), as well as a piano-action keyboard (MOXF8).

I think that single difference is much more “weighty” than anything having to do with sounds, waveforms, etc.

IMHO, I believe that is the most compelling reason to consider owning both.  The difference in sounds is minimal, and there are a few feature upgrades.

I believe most any Demo you have heard that makes the MOXF shine could be duplicated by the XS...in the right hands!