Multicam help.

cameras which are limited to filming in 20 minute segments.

Such cameras are not good choices for event work.  The best option is to just not use them.  Hell, I might even argue that the very best option is for folks to just stop buying them until Canon, Nikon and Sony stop making them that way.  It's wholly unnecessary.  Panasonic's lineup has always continued recording until you ran out of battery or card space, as it should be.

But for those situations where one just has to make due, the PluralEyes plug-in is probably your second best option.  It works pretty well right from inside of PP.

I'm just getting into the multi-cam feature Premiere has; I recently shot a church service where I had 3 video cameras: A Sony camcorder, a GoPro, and a tablet(I had my phone also if necessary). Though I had all those cameras I was only trying to accomplish a two angle/camera shoot.

               The camcorder and the GoPro were my main video cameras; the Sony had the straight on while the GoPro had the angle. The GoPro recorded for most the service but near the end it ran out of power so with a tablet tripod adapter already ready I switched out the dead GoPro with the tablet at that same or similar angle. I later in post I used Media Encoder to stich together the GoPro footage with the tablet footage in order to get one continuous file to use as a single camera in the multi-camera sequence.

But to your question I did a similar thing with my camcorder where the footage was separated in multiple files. You may of already thought about this but I would definitely have a backup camera or few to cover the gaps where your cameras cutoff. The 20 minute limit sounds like you have a DSLR, in which case yeah they have to cool down. But even if the back up is a phone or tablet as I used make sure you have something to cover the gaps where your camera is shut off or cooling down.

               As I mentioned up top, when you get to post afterwards go ahead and stitch together your camera files, keeping organized and straight which cameras shot which angle. Stich together the ones that represent the same camera/angle. once all your stitching is down and you have all your camera angles prepped and ready for the multi-cam in Premiere then from there it should be pretty straight forward to select your camera files in Premiere, right-click and create the multi-cam sequence(select audio for sync), premiere does its magic, then all you have to do is play, watch and select your angles with the number keys as it plays