combine layers

Hi!
I have two layers and I would like to merge them, so that their colors are "combined". If one layer is red and the second is green I would like to create a new layer automaticaly that is yellow.
Is this possible is photoshop?
Thanks a lot.

Put the red layer on top of the green layer. Set the Blending Mode of the red layer to Screen. Link, select Merge Linked or Select All Edit>Copy Merged, Paste.