Let's say you've got two pictures with the same background that can't be merged without the result looking messy:

You can just merge the two, and erase the bits of the layer you don't want to be included.

You just use the 'erase' tool, on the menu on your right.

Click on it, and choose the right brush. (depending on the size of the picture you're working with)
Make sure you erase the right bit though: not the background picture, but your layer!

You can now erase basically every detail from the layer you don't want to be in the end result!
The result?


