Could you explain how these datasets were created?
Both datasets will use their own indices in the range [0, len(dataset)-1].
If both datasets are also using the same samples in the same order internally and assuming thatr supervised_data contains more samples than validation_data, then you could use a Subset with indices = torch.arange(len(validation_data), len(supervised_data)).
However, if the aforementioned conditions are not met, you might need to create a mapping between the samples of both datasets or, probably better, split them during their creation in a clean way.