The bottom line is - if you're happy with the results, then 'Yes, of course you can!'.
But there are differences between 35mm and 120. Film base is different, lenses are different, enlarging lens is sometimes different, etc etc etc.
Therefore, if you find that the results you need from either format are not similar enough, then 'Don't do that!'.
As always, it is a matter of what you need versus what you deliver. Personally, I develop 120 Tri-X together with 35mm Tri-X all the time, same with TMax 400 in the two formats. I get what I need that way. But as Matt says above, that may not be the case for everybody.
Mixing two different films on the other hand, I've done that with no ill effect, but I've heard some claims that it's not ideal, based on the compounds that different films release into the developer during the process. It's not clear to me how much of that has actual substance, and how much of it is hypothetical 'what ifs'.