I've been using 2 SE84UFO2's as mono amps for a while now.
It's actually a simple setup once you've done it. After bridging 2 speaker outputs (L+R) with any single strand speaker wire, it's just a matter connecting the RCA Y adapters.
With the UFOs each having 2 inputs, you need 2 RCA Y adapters per amp if you plan to run 2 music sources.
The Y adapters you need are 2 male RCA plugs to 1 female RCA.
The kit Decware offers shows 2 male RCA plugs to 1 male RCA plug. Decware are longer cables that plug directly to your music source. Both types will work. One of my turntables has dedicated RCA cables so I use the 2 male to 1 female type.
The RCA Y adapters are available for a few $ each for cheap ones up to audiophile grade ones like Decware offers.
For example only, here is a mid grade one (no affiliation):
https://www.audioquest.com/custom-installation/splitters-connectors-adaptors/spl...My UFO amps work great bridged to mono and they drive Klipsch Heresys to quite loud levels. No preamp needed or used.
Hopefully, I haven't added confusion with this post.
John