each card's clocks can be set independently of each other. there is no reason to run separate instances for each card. for example, to configure clocks for multiple gpu, you can use
"--gpu-engine 900" to set 900mhz for every card, or "--gpu-engine 900,850,875" to set 900mhz for card0, 850mhz for card1, 875 for card2, etc.