Each feature needs to be optimized to use more than one GPU. Not all features benefit from computation on a GPU. Newer features such as Focus Mask and Intelligent Upscaling currently support multiple GPUs.
To use Upscaling:
Image > Image Size... > Resample > Preserve Details (enlargement)
Those are two features that are not relevant enough in every day's use to make an overall difference. After a year and several updates I am expecting more in this department.
I truly wish Adobe would stick to improving and optimizing and bugfixing existing parts of the application first and foremost instead of introducing new features which may be nice, but don't pull enough weight to increase my productivity at the end of each day.