Generator sets can be paralleled for either redundancy or capacity requirements.  Paralleling generators allows you to electrically join them to combine their power output.   Paralleling identical generators will not be problematic but some extensive thought should go into the overall design based on the primary purpose of your system.   If you are trying to parallel unlike generators the design and installation can be more complex and you must keep in mind the affects of engine configuration, generator design, and regulator design, just to name a few.