If the PPTP clients are on the outside and the PPTP server is on the inside, then the server will need a one-to-one static entry in the PIX and the appropriate protocols allowed in. Once that's done, then you should be able to get multiple connections going.
If however, the PPTP clients are inside and the PPTP server is outside, and you're doing PAT on the PIX (using a nat/global pair), then that get's a bit harder. PPTP is not a TCP or UDP based protocol, and hence the PIX can't PAT it properly because there is no TCP/UDP port number to use.
If you assign each internal PPTP client a one-to-one static translation, then again this will work properly, but this means you need a valid global IP address for each client.
V6.3 code of the PIX does include support for PAT for PPTP, where it uses the tunnel-id parameter within the GRE packet as the port number for PAT'ing.