how to make one folder duplicate another

Is it possible to have the contents of one folder update themselves to reflect the contents of another folder?

Im embarking on an overhaul of my photo collection so as i might be able to actually find things and enjoy looking at them, rather than just be confused as to what is where.

The point im starting from is to have a master folder with all my 3GS jpgs in it. (I have photos from a camera, but i pretty much stopped using that when i got my iphone, so at this point im concentrating on all the iphone jpgs in chronological order).

Because of hard drive space on my mbp, im storing this master folder on an external drive. Once i had all the images compiled in this master folder I copied it to another external drive, so there are now two identical copies. I now plan to go through the images, throwing into the trash all those that arent worthy of preserving. I have about 4000 jpgs from the last 18months and i hope to trim this down somewhat.

I dont want to have to replicate the 'throwing out the trash' action i take on the first folder when it comes to the second. I just want the second backup folder to update on the fly (or following some instruction). Is this possible. I thought i might be if i used a smart folder, but i cant find an appropriate filter to achieve it.

thanks