Multi-Window vs. camera2.api

Hi.
I urgently want to use 2 specific app-windows side by side (musician-apps . live_bpm and drummers_metronome). Sadly both apps are not capable of this. I have the stock android v. 6.01.
Therefore I tested CM13 et voila - split-screen for the above two apps works flawlessly! So whats the problem?- One other feature that I
strongly want/need... in the stock rom (live-HDR in Camera and a few other camera-features) is now NOT working in CM13 because of missing camera2-api (at least this is the error-message in camera-app)

I found a possible solution myself - download the apk's, decompile and edit the manifest.xml. This should work, but I found no way to download the apks - both are legally purchased by me. The second way, to contact the developers, was wasted. _Both didn't reply.

Any idea how to achieve both features? Perhaps any different rom?

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