This might help:
Export one library to another folder--desktop or external hard drive. Then go into iphoto again and switch libraries--then import all those previously exported photos. Once that is done, delete the obsolete library.
It will save you $30, which is what the Fat Cat software will cost you.
That will not merge libraries - it will only merge the exported versions of the photos form one library into the other library which is not what the OP ask
If you use IPLM you amintain the editing and versions of the photos along with all metadata changes, etc - exporting and imp[orting is a valid, supported function but it is not merging - you can only merge using Aperture or IPLM at this time so far as I know