PIP from Multicam?

(*EDITED... I'm coming back to clarify a few things I forgot when I first replied...)

I stumbled on a way to do this as a newbie also: 

A cool but non-obvious feature of multicam is that you can put sequences in it, not just media. A multicam sequence is really just a nested sequence with some extra functionality turned on. And, we also know that sequences can be nested infinitely. So there's no reason why we can't include a sequence as one of the "cameras" when created a multicam source.

I've had the same exact scenario... I have multiple views of the same event, and use multicam to switch between... that's standard. But, like you, I wanted to also show occasionally more than one camera simultaneously, as a PIP (but a split screen would work equally well). I've done this type of thing many times:

1. Decide which two views I'm going to want to show together, and place them in their own sequence. Synchronize them (manually or with the autosync feature). Arrange the PIP... scale, crop, position, etc. however I'll want it to look. Now I have a sequence showing the entire event with two simultaneous views. IMPORTANT... after you're satisfied, go up to the Sequence menu and choose Render Audio. Then just rename the sequence PIP or whatever. 

2. In the Project panel, select each original media item (all views/audio, including the ones that are also in the PIP sequence) AND select the PIP sequence itself.

3. Right click, and choose Create multicam source sequence. (Assuming you're using audio to sync when creating the multicam... If you didn't Render Audio in step 1, you'll likely get a message saying that a match could not be found and the PIP sequence will be left out.)

Done. Now, my PIP will actually show up as one of the "cameras" and I can switch back and forth between it or any individual view as usual.

Of course there are lots of variations on this theme. You can create more than one PIP/split screen sequence to include to fit your needs. You can create sequences with three or more views on one screen. Really, any sequence, nested or otherwise, can be selected to be included in the multicam source.

The con: I haven't measured, but I would imagine that this could choke slower computers because a multicam already plays multiple videos at once... and now you're playing some of them back twice. ? Not an expert so I can't say that's how it works for sure. But my machine has no issues with this.

The pro: it isn't necessary to plan your entire video perfectly when starting. Since all your views, including PIPs/split screens last the entire duration of the multicam, you can switch at leisure, and go back and change so easily.

After editing the entire multicam and you like it, you might flatten it (or not, whatever). But then you can add dissolves/transitions at all/some of the cuts... of course that's just your taste. But I have View A fullscreen, and then View B appears over it as a smaller PIP, I'd rather not have View B just cut on so starkly (is that a word?).

Hope that helps...

The video below is a multicam PIP (live camera and screen recording). You simply make a cut in the muticam sequence. Then hit the alt key and mouse click on the area your want to have the PIP and drag it into video track 2. You can have a PIP for the full length of the entire video if you want. You can resize as you wish and everything will be in sync. A tutorial for multicam PIPs is next on my to do list.