OK, so I thought the way you do and did what you are thinking and went with 2 x 5500 watts for both the HLT and the BK in a 20 gl batches (28 gl kettles).
Here's my assessment...you won't be sorry. If you are doing all that the incremental cost is split over a bunch of batches. Besides, let's face it...you want it NOW.
Also, you may find (as I did) that it saves big time by doing double brews on brew day, what with the savings of complete cleaning vs. rinsing;plus interweaving the steps for the two batches. So, all the time savings add up.
Can you actually imagine a scenario in which you say (out loud); Gee I wish I did not have all this power and responsiveness. I rest my case.