Multiple GPIB masters

I've always wondered why it wouldn't work.

Apparently it does work. I forgot that I had both PETs connected to two drives, and I had no issues using them that way. I wouldn't expect both PETs to be able to use the bus at the same time of course.

But is there a valid reason not to do this otherwise?

IEEE-488 aka GP-IB is a multi master standard...

Right, but both the masters (two PETs) have the same address. Or don't they have an address?

It isn't quite as simple. GP-IB defines controllers, talkers and listeners. See the overview here: http://se.mathworks.com/help/instrument/gpib-overview.html
Only one controller can be active at a time.