If one is on the front port and one on the back, I would expect them to run on separate power supplies. Just to make sure though ( because it is the only reason I can imagine right now), could you check the energy consumption listed in your USB hub devices?
You can check this in your Device Manager, in the properties of your installed USB hosts.
I made a PSR recording, so you can follow its step-by-step-instructions in case you don't find it straight away. Sorry for the German language, I could not find the language settings for PSR.