If you are using nn.DataParallel, each GPU will get the same batch size (if possible).
If you look at the internals you might adapt the code and use a custom chunking approach.
However, I would generally advice against a system with mixed GPUs.
E.g. your 2080TI will have TensorCores, which could speedup FP16 calculations, while your 1080Ti won’t be able to do it.