Question: As I understand it, you are already using PID to control the energy you're applying to your HLT. What problem are you solving by splitting the 1200 & 800 elements? Why not just use it as a 2k element all the time, and simply let the PID do it's job?
Editing: I'll add this, as my post above comes across as more contentious than I intended.
I suggest that you just join the elements and use them simultaneously because the PID library is expecting, when you go from 79% to 80%, for your HLT to respond to a 1% increase in power. If you're writing a second element HIGH, then a 1% increase in output is going to result in 150% increase in actual energy being injected into the HLT. I don't see how the PID can provide accurate control in that situation.
For what it's worth, I just went down this road with the PID library, using a 5.5kw element installed into a 15.5 gallon (58.6 liter) keg.