Should have no problems. If you check my specs, you will see that, besides Windows 7, I also run 2 installations of Vista, both SP1 (now SP2). I originally had a 32-bit Home Premium version of Vista, with no service pack. I decided that I wanted to try the 64-bit version, so after SP1 was available pre-integrated on retail discs, I bought the Ultimate edition. This came with both 64-bit and 32-bit discs. I installed the 64-bit version with the key that came with it, and then wondered if my key from the original Vista would also work, providing that I selected the correct version (I use the upgrade versions with the double-installation method). There was no problem with this, both versions activated without any problems.
I would expect that Windows 7 will be the same, although whether this will still apply when we have Windows 8 we shall have to wait and see.