I've found a work-around which achieves the desired result, but admittedly it's not as easy as I would have liked Adobe to have made it:
- I drag all the media (two camera files, each with reference audio, and one audio file from an external recorder) onto the timeline of a new sequence.
- Select the media on the timeline, right-click and pick 'Syncronize', using 'Audio' as the syncronize point.
- Delete the audio files which you won't be using.
- On the camera which needs to be upscaled, right-click it and select 'Scale to Frame Size'
- Now that the two cameras and the one audio file is syncronized (and the 720p is stretched to 1080p), select the media on the timeline, right-click and pick 'Nest'. Give your nested sequence a name.
- Your clips are replaced on the timeline by a nested sequence. Right-click it, and pick 'Multi-camera > Enable'
- Use the multi-camera view (Shift-0) to play back the clips, and the number keys to make the edits between the two cameras.
- Then to trim and edit that multi-camera sequence, I have to create another sequence. In the project, right-click on your multicamera/original sequence, and pick 'Open in source window'. You can now make edits from it, and drag it onto your new sequence.
A bit lengthy, but I guess it'll do the job!
Next time I just hope Adobe adds an option to adjust the resolution on individual cameras while they're in a multicamera sequence directly (without having to transcode the original media).