Creo 2 supported installing both 32-bit and 64-bit to the same location during installation. Creo 3 removed this capability and will now only let you install the version that matches your operating system.
The last time I spoke with tech support, the "PTC Supported" solution for Creo 3 was to have two separate installations on the server. They refused to support any type of shared/common/merged installation. With separate installs you would need to either provide separate shortcuts or use a custom script to pick the right shortcut for you.
I really didn't care for this approach since it requires twice the server space and redundant config files. I found that simply doing two separate installations and then merging them together works great. I'd have to double check, but I think I kept the 32-bit version of Creo agent just to make sure it would run on either OS.
There is a free utility, WinMerge that does a great job of showing the differences between the two installs. This is what gave me confidence that this would work. If you excluded everything that is identical and everything that is unique, there are very few shared files that are actually different (at the binary level).
I do still use a custom launch script to set certain environment variables and paths based on the bit-type of the OS. So far this is working great. 
P.S. Simply running the installer for 32-bit in the same location as a previously run installation of 64-bit does NOT work. The last one installed wins. You have to manually merge them.
