I think the only option, at the moment, is to create multiple instances. So, if you want to run a batch, run one instance for each GPU that you have. Set each instance to each individual GPU and increment the seed by 1 per batch, and by 4 (if using 4 GPUs), so each one is processing a different output with the same settings.
Similar setup if you want to produce more passes, but the same seed. Change the per-instance value by one value, and each individual instances values increment by 4x that same initial offset value.
Natively, “we are not there yet”, but I am sure they are working on it. I know a few GUIs do this to create animations. Each “frame” goes to a different GPU. But, only ONE GPU can work on ONE output at a time. Creation, at the moment, is linear. Until they start making “larger creations” that can be made from “areas”, and the results merged back into one singular output. (Like how a 3D rendering program works, in tiny little boxes, to create one large image.)