I recommend that you don't use nuts or chips in the batters. The bumps they create make it more difficult to cut smooth edges. I didn't notice that the brownie mix had miniature chocolate chips in it until I had already opened it, and some of the chips did make for slightly uneven cuts in some places.
To make the heart-in-a-heart brownie: cut a large brownie heart, then cut the center of the large heart with the smaller cutter. Set the smaller brownie heart aside. Cut a small blondie heart, and place it carefully into the large brownie. You can also do the reverse: put a small brownie heart into a large blondie. They're moist enough that they stay together well.
I'm writing the recipe as originally written, but remember to use a larger pan if you're going to cut out shapes.
Baking Mix Blondies
Combine all ingredients in a large bowl and mix well. Spread batter in prepared pan and bake for about 25 minutes, or until center is set. Be careful not to over-bake. (Reduce cooking time somewhat if using the larger pan).