This will obviously depend on your setup. But, if the only difference between the two projects are the textures, and you already have the smaller versions, simply switch out the texture folders between builds. You just have to update the textures in both folders when each texture is edited.
You could also use hard links to create two project directories, and create a bash/batch script that linked the correct files in between builds. If you would like me to expand upon this idea, I could, but essentially, you would be editing one set of files in both directories, but the textures would be duplicated. You would also need to have two sets of of the prebuilt UNREAL files, so you aren’t constantly rebuilding them.
If you are on Windows, http://schinagl.priv.at/nt/hardlinkshellext/hardlinkshellext.html will allow you to create a link for every file in a source directory, while creating new directories and subdirectories. It can do much more, and is free. I use it to move the default Unreal Vault folder on Windows.
There are similar scripts for OS X and Linux.