As I’ve said before, in theory there’s no limitation to having both versions on the same machine, as long as you’re aware that your 2012 software will end up using the 2013 beta background services (Backburner, WiretapGateway, Wiretap Server, etc), so your 2012 software will be affected by any bugs that are present in the beta version of these services. (some of those bugs might not exist in the 2012 or 2012 SAP/SAP2 software).
As long as you’re ok with that risk, you can go ahead and install 2013. Most of the time things will work well, but you might run into import or export issues, since those features will use the beta services.
Hope this clarifies things.
Autodesk Media & Entertainment Support