You'd need to upgrade to preium hosting to increase number of databases.
However, you can have more then one script installed to one database. You'd need to use manual installation though rather then the one-click installer in the control panel. Manual installation would allow you to set the prefix for each installation so they become unique, to have two wordpress for instance you could set one to use wp1_ as prefix and the other to use wp2_ (default would be having wp_ as prefix).