I use two 1500W elements on 20Amp circuits to heat my keggle HLT. I'm doing 10 gallon batches. I am using a HERMS system and I actually heat 17.5 gallons of strike water. 15 in the HLT and 2.5 in the MLT (cooler). It takes a little bit of time, but if you set things up the night before and flip them on early in the morning. They will bring it up to temp with no problems.
Your idea of using the ranco for one and the other as a dedicated on/off would most likely work well. I have mine controlled with one PID and two relays.