"Standard" GPUs (i.e., multiple cards inside the same computer): It is possible to work with multiple GPUs on Isadora, but we really don't recommend it.
The reason for this is simple: if an image exists on GPU #1, but you want to render it on a display connected to GPU #2, the only solution is to copy that texture from one GPU to the other by passing it through the GPU, an operation that can be problematically slow.
eGPUs on MacOS: at the moment, Isadora will work with an eGPU but the performance will be poor. We are working on an update to address this issue. Please stay tuned to our forum for breaking information on this topic. (Written April 16, 2018.)