Seeing Planes Visually that Don't Show up on ADS-B

Yes. You will need a second dongle/antenna for 978MHz, then it’s just some minor configuration. You may want to consider running a second Pi for the second dongle as running two dongles off one Pi has a few challenges (both physically connecting two dongles, and also powering them reliably)

The “official” piaware docs for UAT are somewhat lacking but the forums are a good resource, here’s a starting point: How to Install and Configure Piaware 5.0 SD card image - Quickstart Guide