In answer to the original question - yes, you can.
I have a 160GB drive set aside for Windows 7. I installed 64-bit first, creating the partition and formatting half the drive during setup. After I had that up and running I installed 32-bit with the remainder of the drive.
That's not entirely true.. there's more benefits to using a 64 bit OS than just being able to use more ram.. for one thing, the programs that benefit from a 64 bit OS work much faster and overall more effeciently... if you google the pros and cons you'll see that there is more to a 64 bit OS than just being able to use more ram..
Okay, here's how you do it;
Install one of them.
Start menu > right click my computer > manage > Storage > Disk Management > right click that partition and click shrink volume.
Try to give both partitions equal size
If you're formatting because you've backed all your data up, just delete all partitions on your drive from the Windows 7 disk, then create two equally sized partitions, and install to one, then the other.
Yeah I've partitioned before, I just didn't know if I could install a copy to each of them and boot with it. Do you recommend the partitions be equal size for a reason? I'm probably going to be using the 32 bit version more so I was going to allocate more space to that partition.
Well, considering my processor's L2 cache is only 1MB, I'd imagine that's the reason why the 64 bit doesn't run as well. So yeah the 64 bit would be faster, but with only a processor cache of 1mb and just 2GB of RAM, it's not going to run as fast as the 32 bit. But I still have both versions installed, and I can still save items to both partitions of my hard drive.