Using multiple Picade Player X USB controllers with a PC

I have built several button box controllers for my pc. They all show up and function, however the two that have Picade Player X controllers cannot operate at the same time. If I have both connected only one will work (however both will show up). If I unplug one, no matter which, then the other will work normally.
I was under the impression that these should work as standalone controllers and do not need to go through any other system, and indeed as long as only one is being used, this appears to be the case. Am I missing something here and in fact it is not possible to use more than one as a controller at a time?