Shouldn’t be, I’ve done it before.
In fact, in one of my PCs now, I have a 20x UDMA4 drive on the same IDE cable as an 18x UDMA2 drive. Both reach their maximum speeds comfortably.
Assuming you’re using a modern OS like XP or Vista and fairly recent hardware, of course.
Edit: I’d make sure you’re using an 80 wire IDE cable nevertheless.