As you are in the U.S., why not run dump978 for UAT on the second dongle. I run it on two of my RPIs but don’t see a lot of traffic at the moment.
github.com/mutability/dump978
It takes a few minutes to set up with Joe’s scripts ads-b-flight-tracking-f21/ads-b-receiver-project-setup-scripts-t36532.html
I believe that two dongles on 1090Mhz require two versions of dump0190 and two versions of piaware. I have not seen anyone do it but OBJ has referred to the possibility a number of times.
You can also use the second RPI to test code, cables, antenna and dongles. I have tested Nooelec and rtl-sdr dongles, an airspy(and soon airspy mini), FA Pro stick, FA Amp, FA Antennas, DPD Antenna, Nevis/Hab amp/filters etc. I am waiting on a miniADSB, cavity filter (due soon) and Mode S beast(due at the end of the month).