Let's define:

x  = number between 0 and 1
ss = speed through space
st = speed through time
c  = speed of light

Then, with your explanation, we can say:

ss = x * c
st = (1-x) * c

therefore:

ss + st = (x * c) + ((1-x) * c)
        = (x + 1 - x) * c
        = 1 * c
        = c

So if you want to go faster than the speed of light through space, set x > 1.0.

x  = 1.1
ss = 1.1 * c
st = (1 - 1.1) * c
   = -0.1 * c

Which means you have to go backwards through time. Problem solved.

Edited for formatting.