I have noticed some cases with the external usb sockets (2) have one internal lead with a 9 pin plug that goes on to the MB USB terminal 1 (there are 4 of them ie USB 1,2,3,4.

So therefore the MB's usb terminals can handle 2 usb devices each.

This other case i have has 2 external usb sockets also but with two leads (not one) with half thickeness plugs (4 pin).

I assume that they could go onto the MB usb 1 terminal, but i have plugged them into separate MB terminals ie. usb1 and usb2 and all works well.

Would it save mainboard resources if i connected both half thickness usb plugs to the one usb terminal ie usb1.

Yes, just check your manual for the proper orientation to attach the 4 pin leads. The 9 pin plug is simply 2 -4 pin USB plugs side-by-side. The 9th pin is an extra, does nothing.