Choose backend (GPU/CPU)

So I couldn't find a way to do this in the documentation but I was wondering if there was a way to force dlib to use the CPU if it's built with CUDA enabled? I ask because I'm making a multithreaded application with dlib and only a maximum of 2 threads at a time can use CUDA (because of limitations of the GPU itself). So I'd like to in a thread get dlib to use the GPU in the thread if <2 threads are using the GPU and otherwise fallback to using the CPU.

Also a slightly related question how would dlib cope with a machine with multiple GPUs installed all for compute purposes?