TS2010 is a stand-alone product. It can be installed alongside other products, which I assume includes TS12. Personally, I have 2004, 2006 and 2010 all on the same machine and I've never had an issue.
As far as I know, the patching process does not change, regardless of how many versions of Trainz you have on your system.
Personally, if you already have 2012, I'd suggest that it might be a good idea to avoid the final 2010 patch that enabled multi-player. You already have it with 2012, and avoiding the 2010 patch would allow you to keep compatibility mode should the need for it arise. It would sort of be the best of both worlds.
Patrick