I have a GTX 690 (a combination of 2 680s). Cycles renders it like it is a CPU with 2 threads, so it has two squares running around at the same time, but I have a lot of geometry and it uses the 680s individually. Is there a way to combine the power?
That is the expected behavior: when you use multiple GPUs, blender renders as many tiles as there are available GPUs (if blender has been confugured to use them).
The key concept is that each GPU is rendering a portion of the image using its own resources, not combining them. In other words, Multiple GPUs will render in parallel, each of them using it's own memory pool.
If you have 2 GPUs your image will render in almost half the time. 4 of them will cut render time to 1/4