You can say that you have 3x2 speeds and you gear as follows:
Ex 1 In 1 = 1
Ex 2 In 1 = 2
Ex 1 In 2 = 3
Ex 2 In 2 = 4
Ex 1 In 3 = 5
Ex 2 In 3 = 6
(Ex = External = left shifter ---- In = Internal = right shifter)
It may sound difficult but it isn't while you actually shift so there should be no problems.
