On smaller engines there is no "rear bolt" but on some there is. 20 hp may qualify. Do you have a zinc trim tab at the rear of the antiventilation plate, behind the prop? If so, unbolt it and remove it, taking note it's lateral position so that when you put it back, your steering will be as it was before....not pulling to one side or the other.
Look up in the recess under where the tab was mounted for a hex Allen bolt. If you have all this, removal of that bolt should grant your wish in separating the sections.
If not, then you must have a frozen drive shaft to engine crankshaft interface, which can happen especially if the drive shaft is carbon steel. I don't know when Merc gave up on that ridiculous design flaw and went like OMC had been doing for years, with a corrosion resisting stainless steel drive system. If the latter here is the case Google separating the lower unit from the mid section on outboard motors and you should get a lot of answers, many of which are addressed right here on iboats, or go to the iboats archives.
Good Luck