Multiple USB3 Busses, OSX

I’d be grateful for any help on this one. I would like to install two 10-port USB hubs on my iMac (OSX 10.9.4, 3.5 Ghz i7, 16 Gb) but I’m unsure of the viability of this. The CPU has four USB3 ports, but I’m pretty sure they share a single bus. I’d like to be able to mount numerous 7200 RPM hard drives, though rarely would I be actively using more than one or two at a time. Assuming I’m not attempting to change a phone (which I gather maxes out the current more quickly) is this practical, and if so, do they need to be the same manufacturer, and do I require a particular firmware or chipset? Thx