Hi @DUPONDIUS
can I ask why manually?
Btw, I don't know how strong is your background in SCRIPT (the language) and locking/unlocking scripts, anyway my suggestions would be:
1) don't do it manually unless you have a huge amount of time, patience, concentration.... but to be honest, even with them I'm not sure your idea is feasible
2) if you have a so strong commitment to that, try to understand as deeply as you can the context of your challenge. I'm sure there are a lot of better and more complete sources, but if you want here it is a recap I did some times ago about relations occurring between various locking/unlocking scripts (I think its usefulness -if any- relies on being a quite compact summary, maybe helping you not getting lost while you go deeper thanks to more authoritative authors):
https://medium.com/@baro77/btc-locking-and-unlocking-scripts-cheatsheet-ff6d515447dePS
Just one more thing. When I read:
We would take the private keys from both addresses (public key) and we would have to combine them.
(private key 1) & (private key 2) = multi-signature address (P2SH)
the word "combine" reminded me about linearity of Schnorr signatures... always because i don't know your previous knowledge, were you thinking to something like that? If not, never mind.