I've just upgraded from an X5 to an X51v. My wife has expressed an interest in using my X5 but what I want to know is whether I can sync the two from the same computer.

All the FAQs I've read seem to say Activesync 3.8 for older devices and Activesync 4.x for WM5 devices.

Anyone tried to just use one version? What happened?

Can two versions of activesync live happily on the same PC? Methinks probably not!

Any suggestions?

Bartender! Mine's a Bottle of Dog!

No, two versions can't be on at the same time.
Newer versions of AS also work with older OSos (such as the one on the X5).
You can sync both using 4.x.