Do you maybe know, when frames rendered by GPU 1 (e.g. fast GPU) need to be sent to GPU 2 (e.g. slow GPU doing the compositing and outputting to the monitor), how does the data actually get transferred? I can imagine the following possibilities:

1) GPU 1 writes to CPU RAM, GPU 2 reads from CPU RAM

2) GPU 1 writes to GPU 2 RAM via PCI Express (DMA between devices)

3) GPU 2 reads from GPU 1 RAM via PCI Express (DMA between devices)