http://www.aemelectronics.com/files/...tion%20Kit.pdf
Page 5
I "think" you have to change the ECU address and PID on the second one to something different but I'm not 100% sure of that.. I know at least one of those settings has to be different, possibly both.. but playing around with those should get it going. I think anyways.
Basically have to tell the 2nd unit to use a different PID, and [likely] a different address also, so that HPT can differentiate between the two readings/units.. I don't think it really matters which address/PID you use, just that it's different than the first, but I've never tried it so let us know how it goes..