Your PC will work with one at a time and the speed it goes will be in relation to the memory available. You may notice a time delay with a PC with low memory but it should not be a significant difference. As you probably can not physically change the two at once (unplug and plug in) your PC might even complete the first task before you can action the second.
As I said before, the PC will only handle one job at a time so while you will be running two USB's the PC will only load one at a time. Each machine varies but it probably could manage 8 or more at a time.