Building on what cbrandtbuffalo said ...

I'd probably either seperate this into two seperate forms (if the user can figure out which input to use, they can figure out which form to use)

JavaScript is a possibility, but then you run the risk of what to do if the user has it turned off.

Ideally, you could just send a redirect (302) to the other system, but because some browsers handle 302 as a 303 (doing a GET on the location you're trying to redirect to, rather than sending the POST again), this isn't reliable, either.

Your last possibility is having script1 proxy the connection to script2, but I personally don't think this is a particularly good move in the long run (unnecessary load on server1, and makes script2 dependant on someone not messing with script1 and server1)