I know The Pol has a heating element installed in his. I don't use a cooler as a HLT, going with a sanke as well. I do know people with gravity-gas systems and 2 coolers will heat their water and then put it in the HLT cooler to hold it.
If you are fly sparging then a 3 pot/cooler/whatever system is very nice. For batch sparging you can get by with a boil kettle and mash tun. Just dump the first runnings into a brew-bucket while you use the rest of the water (or two sparges) to sparge the grain, then use the boil kettle for actually boiling.
"People should not be afraid of their governments. Governments should be afraid of their people." - V
Secondary: Shady Lord RIS, Water to Barleywine, Pumpkin wine, burnt mead
Kegged: Crappy infected mild
Bottles: Apfelwein, 999 Barleywine, Oatmeal Stout, Robust Porter, Robust smoked porter, Simcoe Smash