I just tested this scenario and was able to establish a TCP connection between two PPTP clients that were connected via sererate ISPs.
Ok... thanks a lot for trying that!
I bet in the time it took you to make this post you could have tested this yourself.
The case is that i have a netvork, where clients NOT should be able to see each other!
I did test it, but could not get connection - and was happy, but then i got spooked by a friend that argued for that it had to be possible... so now im just trying to get third opinions and documentation.