Unfortunately that's not possible with Electrum (and most modern wallets, actually). Electrum is a deterministic wallet, which means that it calculates all of your addresses from a single seed. As such, it's not really possible to "delete" the private keys to a single address.
You could export the private key for a single address from wallet A and import it in wallet B, making its funds available to both wallet A and B. However, you cannot prevent the original wallet A from also still being able to access the funds.
Thanks a lot!
Your explanations were very precise and helpful! I guess you answered my questions ^^
Well it's a bit of a shame that I can't simply make the transactions myself but... But what you're saying is still good for me! It means I can "regroup" wallets as I would do with addresses!
I might want to look only for labelling my addresses better though because that doesn't sound like wallets were developped with this purpose xD
Thanks again man!