2 wordpress installations/one website? Possible?

No reason why not.
Separate folders, seprate table prefixes.

Yep, shouldn’t be a problem.
If you don’t have two MySQL databases, just do the differing prefixes like podz suggested.

How do I add a prefix to a table via myphpadmin?

you don’t ……..well, you can by editing the post table name, but if you are installing wp, edit the table prefix in ‘wp-config.php’