If you copied the steps in the tutotial, you should have:
<JP AXIS="1,0" M1="0" M2="1" MV="Infinity,-20" /><JP AXIS="0,1"M1="0" M2="1" />

In this example, M1="0" is the threadmill / booster, and M2="1" is the static ground.
To add another threadmill / booster, copy the code and replace the threadmill (M1="Z value") with the Z value of the threadmill/booster you want to add, but leave the static ground the same.

For example:
<JP AXIS="1,0" M1="2" M2="1" MV="Infinity,-20" /><JP AXIS="0,1"M1="2" M2="1" />

In this code above, the second threadmill / booster has Z value 2, and thus M1="2".
This way you should be able to add as many as you want. Though make sure to leave the static (M2="Z value") the same.