Two albums or two Zenphoto installs?
If the first: You can either move the images from one album to the other via ftp. or use the move/copy feature on the admin edit page. The latter will also move the data like descriptions you have entered on the backend.
If the second and two installs are on the same server you can just move the albums/image from the `albums` folder of one install to the one of the other. Zenphoto is filesystem based and will discover them. But note the data you entered on the admin backend will not move with the images unless you added them as exif data in the first place.