Yes, they run on the same system without any issues. LP9 is titled "Logic Pro" where LPX is titled "Logic Pro X", so there is no conflict in the Applications folder. You will need to update to the latest version OS X 10.8.4. As you know your 32-bit plugins won't run in LPX. But I feel any plugin worth having should have already been upgraded by the developer to 64-bit.
After having used LPX for two days solid, I'll never go back to LP9.
But yes, they both live in your Applications folder, side by side, quite nicely. Just can't run both at the same time, as you know.