postgresql installation problems

Probably the following might have been asked before, but I've been
searching the web for the following problem for 2 days already:
I need to install an end-user application which demands 2 postgresql
users each owning its own database.
One user+database is used for a data management system (I call this
one GUS), the other for the front-end application (a data-analysis
program), I call that QPCR.
So I installed postgreSQL according to the documentation. A server is
running on port 5432 now.
Now the question: when i try to install the application, the
installation demands to set the port for each user separately. When i
try to use 5432 for both databases an error pops up with the
notification that that port is already used by QPCR and therefore
can't be used by GUS. How do I set a port for each user separately??
Do I need to install two clusters (and how do I do that?)

I hope someone can help me with this.

Thanks already,

Reno