If you're looking for a meeting place or a good stop around ½ or halfway between Waynesboro, PA and Union, you can try Pottsville, PA, which is about 2 hours northeast of Waynesboro, PA, or almost 2 hours west of Union.
About ¼ or a fourth of the way, you can stop at Harrisburg, PA. This is almost an hour northeast of Waynesboro, PA.
About ⅓ or a third of the way, you can stop at Hershey, PA. This is a little over 1 hour northeast of Waynesboro, PA.
About ¾ or three fourths of the way, you can stop at Bethlehem, PA. This is about 2½ hours northeast of Waynesboro, PA, or almost an hour west of Union.
If you drive straight through without stopping at all, the total distance is 220 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 3 hours, 32 minutes.