Way too much in the title I guess.  I have a cylindrical assembly that I need to eventually mold.  I got most of the external work completed and now need to work on the interior of the cylindrical shell.  I would like to cut the assembly into two pieces (top and bottom) so I can add ribs, supports, mounting area for switch, etc., and then recombine later for a finished piece.  I've tried various help searches and youtube searches, but none give me what I need.  Of course, I am so new to this software, I may not even be asking the right questions.  To start, how do I cut an assembly in half along its horizontal axis and save both pieces as separate files?  I'm sure I'll need help with lip and groove later, but for right now, I am just stuck. 

Thank you