In case you don't have them, you can see the installation notes here
Bring the cable (black, white, ground) coming from the T-stat into the junction box that's a part of one end of the heating unit. Wire-nut the two grounds together.
The wire from the safety cutoff should also be black. So you have two blacks wire-nutted together. Remove the wire nut.
Use a wire nut to connect the white from the T-stat cable to either one of the blacks that you just separated.
Use a wire nut to connect the black from the T-stat cable to the other black that you just separated.
- Grounds together.
- White from cable to one of the blacks from heater
- Black from cable to the other black from heater.
Run another cable from the junction box to the second heater, and repeat the above steps.