"Time stretch" is the wrong solution. "Vari-speed" will work better.
Each machine recorded (say) 44.1K samples per second, but they didn't agree on how long those seconds were. Now you're playing them back from yet another machine which probably doesn't agree with either of them, which means that neither will play back at exactly the same rate it was recorded, but one will play back faster than the other. You need to slow it down with all with the pitch shifting that entails in order to really get them in sync and in tune with one another. Time stretching without pitch change is a form of re-synthesis and will just mess it up worse.
This is pretty easy in Reaper by alt-dragging the end of the item until it actually lines up. You have to make sure that "Preserve pitch..." is disabled for the item, and it helps to trim each item to start on the clap at the beginning, line them up exactly, and then glue them each or else vari-speeding will move both claps, and you'll be going back and forth re-aligning the beginning every time you try to adjust the end.
This is easier with digitally recorded stuff because while they might not agree between units how long a second is, they will all be pretty consistent internally - it won't drift or wobble like tape or other analog media might. Analog wow and flutter make it damn near impossible to really re-synch things like this, but with digital it can work fine.
I had a chance to put Audacity's 'Change Tempo' effect to use on one of the tracks, hoping to making it exactly the same length as the other, but with no success. (I assumed 'Change Tempo' is the equivalent to Reaper's 'Varispeed' capability.)
I imported one track into Audacity, pasted the other below it, selected one of them, entered the 'Change Tempo' menu and set the new length to match exactly the other. After about an hour of processing, the selected tracks couldn't be synced, as drift was still evident. I've never used the function before, so it might be I was doing it wrong, or that the two functions are not equivalent after all.
My next experiments in software syncing are going to be a bit more scientific than this, maybe even with a remote to start recording on both machines simultaneously (if I can get one compatible with both machines).
It also appears that the drift I'm seeing is not linear or even constant (a bit either way here and there, with the net drfit being about three seconds by about 02h20m — which would entail custom editing specific segmemts of the recordings. But two hours was much too long to begin with.
It's too bad Reaper isn't available for Linux...It's solved a lot of problems for a lot of users. But Linux is what I'm stuck with (I got a modded Raspberry Pi with a better DAC for audio editing/listening because I like fanless computers when listening), and Audacity is the only decent audio editor available on that platform.
It also looks like I should be using the Zoom as a nexus for multiple microphones, instead of using separate and unconnected recorders, a setup which leads to problems like this one.