I have purchased Windows 7 Ultimate with two DVDs 32 bit & 64 bit with one key. I wish to try 32 bit first & 64 bit later. Can i use same key for 64 bit software later on same computer?
As long as it is reinstalling, on the same computer moreover, you can use the same key.
32 bit or 64 bit edition makes no difference in this matter.
Edit: Note that if your computer is capable to use 64 bit, there is no actual reason to try the 32 bit. They are about the same, besides that 64 will allow you to use more RAM (over 3gb), and some programs will have improved performances. But besides, there is no functional difference between the both. Eventually, if you use some exotic hardware, you will have difficulties to find drivers for a 64 bit system, and that would eventually be a reason to stay on 32. But other than that, I don't really see a reason to install 32.
If someone could add on that in comments, it would be nice, as I could eventually be wrong about some details.