I want to be able to switch the sensor data being sent to the HMD from the PPT

[v-Salik]

I have two sensors: ppt1, ppt2. I want to press a button and switch which sensor the HMD reads.
So what I do is define a function swap:
def swap:
temp = ppt1;
ppt1 = ppt2;
ppt2 = temp;

Now the problem is that the hmd still gets its position data from ppt1 regardless of whether I change the variables or not how do I override this?
Thanks