Ideally, both CPUs in a dual system should be identical, not just in speed, FSB and cache size, but also in revision number (called stepping). There are a number of bugs that could occur if different versions of the chips are used.
The simple way around this is for a motherboard to insist that the 2 processors are identical in every respect. If you're running a server, this is probably a sensible precaution.
If you look in Intel's technical documentation, you can find out exactly which steppings are compatible with other steppings. In fact, there are no known problems with mixing any P3 933 slot processors - conversely, there are some troublesome combinations of socketed 933 processors. If you want to look for yourself, you can find the full document
here. Even if you don't want to look at the compatability tables, there are some helpful pictures of where to look to find out the revision numbers.