Windows 7 install both 32 64

Theoretically, yes... since it's the same computer.

But... Why would you want to do that? Win 7 x64 runs 32 bit code just fine.

A software developer might want to test programs under both 32- and 64-bit versions of the OS. In fact any decent software developer certainly would want to do that.

Absolutely true.   I asked, though, because it's seldom we have developers here, and because a (professional) Dev would more than likely obtain same though work or via MSDN subscription. Under those conditions, the licencing is irrelevant from a user standpoint: Either the company paid for it, or it's included in yoru subscription and you can use however many you need.

Because not all sw and hw will run on x64

Not all developers are professionals who subscribe to MSDN. I do a little programming, but not so much that I could justify the expense of an MSDN subscription. I've got lots of OSs installed on a few PCs.

Just to put a different spin on things: If you're referring to the retail box that ships with both 32 and 64bit discs, then no, you only get one product key and can only validate one of the two. If you buy them separately, like the OEM version, you should get a product key for each.

All 32 bit software will run on x64 right now - the exception is something that needs 32 bit drivers or that has a 16 bit installer. Those are getting more and more rare, so honestly, I'd just go with the pure x64.

None of these responses relate directly to my initial question, though;

If you install BOTH versions (x32 and x64) on the SAME pc (but on separate partitions, of course), will you be able to get both installations VALIDATED by Microsoft???
Anybody tried yet?

See my post. How does that not answer your question?

Sorry, I overlooked it  
Your reply perfectly answers my question.
(But - can we be 100% sure on this? Cause, you CAN reformat a drive and re-activate, can't you?)

According to Microsoft you can activate both versions, but only one at a time. So you can't do what you want, but if you make the wrong decision you can always change your mind later.

Clarifying, thanks all.