With the elements I'm using, I whack two of em in the boiler until it reaches the boil and pull one out. I have to run them on seperate circuits, but I have a long ext cord so its no biggie.
I find one of these elements is enough to maintain a nice rolling boil without boiling over. When I use them for my HLT, basic acquirium temp controller off ebay for approx $40 delivered works fine. The only thing with them is you need to spend a little on a box and wiring if you can't do so yourself, so it may be easier to by a temp mate from C'brewer if they sell them ready to go (can't remember if they sell in box).
I've been getting by with just one for my boiler, it just takes a little longer to bring to the boil. Recently got another one from BeerBelly for $50.. works a treat!