I love to make a stacked pumpkin topiary every year for the front porch sometimes with three pumpkins - sometimes with two. Here are the steps I followed to make mine:
1. Fill urn with dirt or gravel
2. Break the stem off of the large pumpkin and position it in the urn
3. Stick some wood skewers in the large pumpkin with the sharp side facing up - I usually use 4
4. Position small pumpkin on top of large, gently pushing it onto the exposed skewers. This will stabilize the topiary and hold the smaller pumpkin in place.
5. Hot glue pieces of dried moss in between the pumpkins for a finished look. You can also use fall garland or raffia.