G'day from Down Under, Seth...

The bad news (from a financial standpoint) is that all wireless mic systems (like your current one) require a complete "chain" (a transmitter sending to "its own" receiver) whenever two sources of sound are to be heard at the same time, each being "added to the mix" and individually controlled "downstream".

If, however, your requirement is for just one mic to do most of the work, but then swap to another, then (unlike analog systems, such as your current one) the "digital hand-shake" of all Line 6 XD-V systems allows an almost instant, amazingly silent ("pop", "click" and "shash"-free) change-over from one transmitter to another by simply turning on the second and then turning off the first, ...with both having been set to operate on the same channel.

Regarding V55s vs V75s, the "key" difference for me (other than those you've noted) is lack of Line 6 "Monkey" USB upgradeability in the (approximately one third) cheaper V55.

That "future-proofing" option (along with the enhanced RF diagnostics, inbuilt antenna distribution, and many other upgraded features) is pretty much the main reason I invested in an "extra" XD-V75 "kit" to supplement and support the two racks of four XD-V70s that I routinely operate.

I feel it has proven to be a very wise investment indeed.

Feel free to click on my pink avatar for details of my location rig.