I have a 3D ducky mold to make a topper. I've never used one before. It is fairly large size. Can someone tell me what is the correct way to join the two halves? Should I press them together while the clocolate is still soft? If they harden first, how do I get a nice seam where they join? Really need some help on this. Thanks.

Do you have the wilton cake pan mold or something else?

Best way is to make each half separately. Turn a baking sheet upside down on your stove burner and heat it briefly. Slide both halves of the duck around for a few seconds on the warm pan. Quickly put the halves together while the choc is melted. Use your finger to smooth out the seam.