Multiple databases one directory

Hi!

Another fun question in our ongoing analysis on whether to switch from
mysql to postgres. (Just as an update, Postgres has performed
flawlessly on all of our stress tests so far.)

We have a situation where we will be creating two fairly large and
complex databases with many tables (thousands) each.  From what I
understand, postgres keeps everything in one big data directory.

Would there be an advantage to putting each of the two databases into a
separate directory and starting two instances of postgres?  Is it
better to just lump everything together.

In a perfect world, we would buy another database server and raid for
the second database, but being a small company, we just  don't have the
budget right now. The raid on our current server is much bigger than we
need.

Thanks,

-N