I'll apologize to Howard up front -- I learned by trying to emulate Flea's playing...and I've enjoyed it quite a bit

But I'll back his basic point that it's not a very efficient way -- watching numerous great bassists who slap, you usually see them with the bass up high on their body, and their thumbs parallel to the strings -- which really makes sense because then there's virtually no chance for your slapping motion to hit any other strings.
That said, you can adjust your form to this technique, or develop a technique that adjusts to your form.
A pointer I received recently was very interesting and helpful to me: slap the strings over the fretboard, not over the space between the end of the fretboard and the first pickup. So basically your thumb is slapping at the 22 fret or so. Your popping fingers should still be in the gap, so they shouldn't have any problems. I like this for a few reasons -- it gives a really bright ring from the strings hitting the frets and the string stops when it hits the fretboard, so there's less of a trampoline effect on the thumb (something that was challenging to me early on). And as was said earlier, you hit and retract the thumb as fast as you can.
If you play with the bass low on your body, slapping the higher strings can be a pain, and even worse if you play a 5 (or more) string bass. But anything can be done if you put the work into it!
The problem I always had was I couldn't come up with different slapping patterns to practice, so I always practiced a simple T-pause-T-P-T-P (with two octaves with the fret hand...like e3 - d5). But the guy I've been getting pointers from recently has started showing me new ones -- and since he can play them fast as the bejeezus, I can hear what the ultimate goal is -- which helps a LOT. One he gave me recently is really simple, but, obviously, takes a little practice to get the coordination down. All the tones are muted, so its really a bakitta-bakitta-bakitta kinda fill noise. I fret the g, d and a strings with my index finger, and get my other three fingers ready the slam down on the fret board one fret higher on the e string. Then the right hand motion is: Pop the d string - hammer the fretboard with fret hand, and leave the fingers down - Slap the e string. Done quickly, you get three percussive tones. But it's also a motion that you can get up to a very fast speed, and then it becomes very useful as a bridge pattern in the middle of a solo, or some fast riff, or just as a warm up exercise.
Hope this helps! Cheers!
Jeff