So the manual says 2.4 is video downlink, and 5.8 for aircraft transmission; I don't care much about video as I keep it running when I start, but RANGE-WISE, I only need a 5.8, yeah?
If you just want to extend the control range then yes, just one 5.8 amp is fine. But this may not be that useful. Without video you would need to keep the drone in sight, and it's difficult to keep the drone in sight when it's more than a few hundred feet away - which should be within the control range of the stock RC.
-Adrian