I mean you cant be writing to the same database from 2 seperate installations of vbulletin.
I guess in theory you can. One uses table prefix the other one uses a different table prefix or non at all.
Technically I can't see any reason why two versions of vB could not use the same database, I assume you would need two licences to do it live, but the database itself would not care. I have done it in the past on my test system, running different 3.0.x systems in parallel to check & port hacks when upgrading.
I don't think any 3.5.0 table changes would stop 3.0.7 code from running, but the results may be a bit odd as all the templates have been updated, and the version number would display as 3.5.0. probably not a good idea, but possible in theory.