[Deputy]

I'm guessing no, since they are sold as seperate items. And I bet any money if a guy with a P88 could use the P99 mags he'd jump on them. I saw a P88 mag selling new in wrapper for $125

Oops...looks like I may be wrong. Midway is selling a P88/P99 combo mag...
http://www.midwayusa.com/midwayusa/applications/eproductpage.exe/showproduct?saleitemid=521712

From responses below looks like only the extended mags would work in both and ProMag is not a good choice.