overlay between two Equirectangular/Spherical projections

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Hello,
I should realize an algorithm/function (in C++) to overlay two Equirectangular/Spherical projections like the following:

pano1
pano2

Can you address me in doing this?
Does OpenCV already contain something that may be of help?
Naturally Verticals should be preserved while searching the best fitting and modifying just one image (pano2) to overlay in the best way over the other (pano1).
If OpenCV can't do this alone I'd really appreciate if you can anyway point me to the right path.

Thanks in advance