uh, aren't you already running two elements? one in the BK and one in the HLT? you just can't run them simultaneously right now.
rough calculations from sunday's brew day (5500w element, 1 at a time)
need 17 gallons total, but can't heat all of that at once.
start with maybe 12 in the HLT at 50F (ground water).
50F -> 165F = 39 minutes
then i mash in, add more water to the HLT and heat it enough to get the HERMS to the right temp plus sparge. it's done at about the time i'm done mashing in and stirring. (10 min)
mash out at 168 (about 10 min to ramp via herms)
add to boil kettle. sparge.
13.5 into the kettle around 165F
13.5 gal 165F => 212F = 18 minutes
total water heating time: 39 + 18 = about an hour
2 x 3500W elements. can be run simultaneously
i need 17 gallons so i split it into 2 vessels (HLT + BK) and get all my water at once!
50F -> 165F 8 gallons = 43 minutes [4 minutes over my last brew day]
BK water goes into the mash, and i'm instantly able to control temps with herms, no lag. [now 10 minutes up on my last brew day]
at the end of the mash, ramp the 8 gallons in the HLT to mash out temp. [net result same as before. the herms will still take the same amount of time to ramp the mash]
13.5 gal into kettle. 165F to 212 @ 3500W = 28 minutes [10 min longer than last brew day, net result is 0 minutes longer total]
with this method, however, i can be heating water in my HLT for my next batch just as soon as it's empty. with the above method i would have had to use propane or some other method to heat the second batch water.
so the net result is basically a wash... and can be shortened by simply starting the BK as soon as there's enough liquid to cover the element. i didn't put this into the calculations because i couldn't account for the math as easily.
the answer to this particular question is technically MORE POWER, but not the way you think. by expanding your total heatable volume into 2 vessels at 7000w total you can heat water quicker than with 1 5500w element, and also free yourself up to double batch days.