Two Postmasters on the Same Machine

I want to run two versions of PostGres on the same machine, but on
different ports (I currently have 6.5.3 installed, and I want to install
7.0.2 in a different directory and run it on a different port until I can
get all my code ported over.) IS there any problem with doing this? Will
the two postmasters attempt to use each other's shared memory or do any
other strange things? The only thing I can think of is that I'll have to
specify the PGDATA path on the command line for the 7.0.2 instance, as the
PGDATA environmental variable will hold the path to the 6.5.3 data dir.

Also, are the 6.5.3 libraries and 7.0.2 libraries compatible, or will I
need to compile second versions of PHP and the Perl DBI module as well?