Multicam edit a single 4k video with proxies

I think we need more info on this "two camera cut effect." Can you post a screen shot or rough sketch of what you're trying to do? Is it like a picture in picture and you need both to be multicam clips? I will tell you, there is no reason I can think of to create muticam sequences out of merged clips, or to nest a multicam inside a multicam; so there's probably a much easier solution to what you're trying to do.

Otherwise, here is a standard workflow with multicam/dual system sound and proxies. Run this first, then we can deal with your intended edit:

1. Import all your A7 clips and external audio into a new bin.
2. Select ALL of them, right-click and select "Create multicam sequence..."
3. In the next box, give your clips an identifier (like "synced"), choose the sync method – Timecode if camera and audio had jammed timecode between them, otherwise Audio, sequence preset to Auto, audio channels to mono, and check "Move source clips to processed bin". Then click OK.
4. Premiere will do all the work - it will check the timecode or audio waveforms and match up the clips accordingly, mute the scratch track from the camera, then create multicam source clips in the bin. All shots that got synced go into a new Bin called Processed Clips. If you get a warning that some shots were not synced then you'll find a few stragglers in the original bin – maybe false takes with no sound, or anything it just couldn't match up.
5. This is the preferred method for syncing dual system sound and picture – regardless of how many cameras you have. It keep the original timecode intact and allows you to edit with these new clips just like any other clip, plus you can add things to the original clips - like an effect in the Master Clip effects – and it will ripple into the multicam clips. Merged clips, on the other hand, destroy with the original timecode and are a nightmare for things like post sound finishing. It's an old method and should be avoided.
6. Now, go back into that Processed Clips folder and select all of your original camera clips. Right-click to create proxies and go through your preferred method. If you need further help with this I can point you toward some tutorials; I personally make my own settings here that make small, 720p ProRes Proxy files.
7. Those proxies will work with those multicam clips, because they're really just nested sequences containing the original camera clips. If there are proxies attached to them you will see them when the proxy switch is toggled. 

HTH,

WM