I’m sorry for the late response!
the linked heart cookies were achieved as follows:
bake your two cookies and immediately after they come out of the oven make a one inch cut in one cookie. I did it where I pictured the linking would cover it up. I didn’t want more than 1 inch cut because a larger piece joined may not be as stable.
I Decorated the solid cookie and left it to dry overnight. The next day I linked the two cookies and used royal icing to glue the cut cookie closed.
you want to have your cookies elevated and supported. let the glued cookie dry for at least 3-4 hours to make sure it’s dry.
to decorate the glued cookie you want to elevate it while supporting the finished cookie upright.
I chose a simple design because it’s tricky decorating the glued cookie under the finished cookie.
hope this helps.