No, there is no way to run a user function on multiple GPU at this time.
At this time, each process can use gpuDevice to select a single GPU (this is done automatically for parpool members when the pool is no larger than the number of GPU.) Selecting a second GPU device would reset the first device.
Mathworks is doing work on sharing load between two hardware connected NVIDIA, and has implemented it for one particular kind of deep learning, and for one other task that is not coming to mind at the moment. Unfortunately I am having difficulty finding the appropriate postings.