It's very common for ARM systems to have disparate processors. My GP2X handheld is dual core system. It has a regular ARM processor and then a second processor for accelerating some types of multimedia functions.
Then, of course, there is modern x86-64 systems were the GPU is a now a coprocessor that can be used for anything, rather then something dedicated just for graphics.
I don't know how common it is to have disparate general purpose processors, however. With my GP2X the application had to be optimized to take advantage of the second processor.