Finally you can duplicate the newly merged layer and use the two halves to make a complete love heart.
Click the Duplicate Layer button and then go to Layer > Flip Horizontal. Now click on the Move Selected Pixels tool and use the left and right arrow keys on your keyboard to position the layer to make a love heart. When you're happy with the positioning, click the Merge Layer Down button to combine the layers into a single layer and there you have a love heart drawn in Paint.NET.

