I don't know, I don't run IIS. In apache, you'd tell it to parse any file extentions with *.php through both php and asp. The php engine would work over the php side of it, and the asp engine, the asp.
You could get really funky with this, if you wanted, by making sure one parses first ( say, the asp ) and have it write out php, to be parsed next.
You can't, however, have these two languages communicate back and forth. You can do one way communication only, sorta, and that's it.