Simple answer here is buy a Mac and your issue is solved.
On a Mac you can setup an Aggregate Audio device in Audio MIDI Setup, then you would have your solution.
On a PC, this is limitation of the ASIO SDK itself developed by Steinberg, it only allows one device present at a time.
If you wanted to combine your interfaces, you could run ASIO4ALL to pull your interfaces together, however ASIO4ALL relies on the WDM/KS / MME drivers and does not provide you your low latency solution.
Even if Studio One had the ability to handle multiple ASIO devices and in OS X with Audio MIDI Setup, you'd still have to deal with the inherent latency differences between the interface drivers from each manufacturer.
You'd be better off trading in your two devices and getting a different I/O solution with the channel count you need than trying to combine interfaces.
