It wouldn't be double DACing, it would bypass the V20 DAC if you were to connect it via usb.
That sucks, because I think double DACing just sounds like a wicked awesome thing to do
But, there you go, certainly the Mojo DAC would out perform the V20's DAC, yes?. I'll, of course, play with it both ways when I get the adapter.
On a tangent note, I love my V20. I have the S7 edge, Moto Z, iphone 7 and V20 on my plan, and I prefer the V20 to all of them. (Songs of Lament for the Note 7 I was forced to exchange). The extra speakers, DAC, large screen, removable battery, push button removable back cover, multi option always on display and wide angle camera really do it for me. I bought it for my business cell for the extra clarity of the speaker phone, but end up using for personal use and play, too.