When I cook a whole pork shoulder (picnic and butt), I usually let it go until internal temp reaches 195 degrees. This way it will pull easily (both portions). If you don’t want to pull it, then you can slice it just like ham (as mentioned above), or do the following:
Make an incision parallel to the bone. The incision should be deep enough to hit the bone. Once you hit the bone, make smaller incisions along the sides of the bone until its easily removed. At this point you can split the picnic in a half or leave it whole. Put the flat surface (where the bone was) against a cutting board and slice it thinly. Enjoy!
I hope this helps.