3.07 and 3.51

[Lionel]

I am currently using a heavily hacked 3.07.
I'd like to buy another license of 3.51 for a different site.
Both sites are to be on the same server and could share the same db if needed, 3.07 has no prefixes.
I could import my 3.07 users into new 3.51
My question is: will it be possible to have both running with same user base? I only the user and passwords and perhaps profiles.
After import, I was planning to insert new user registering in 3.07 into 3.51 also and vice versa. And do the same thing for password changes.

Is that feasible at all? How would I go about that?

Thank you.