Is there a way to select matching pixels on 2 different layers?

Hey there,

I've been wondering about this a long time now and can't seem to figure it out. I hope u guys can help me crack this.

If I shoot 2 different portraits (of 2 different people), against the same background (a complex one not a backdrop), using a locked frame on a tripod, giving me the same exact background in both pictures, with only the portraits changing.

If I then take these 2 pictures, and create layers out of them in the same Photoshop document, and align them perfectly on top of each other.

Is there a way (action, plugin, script, etc...) to select the pixels that did not change between the 2 layers? In other words, the pixels that create the background of both pictures?

And would it make it easier if I shoot the background alone, and then the portraits, and then follow the same steps?

I'm posting this, and crossing my fingers.

Thanks,

Pierre.