I bought a second Darbee not necessarily to use together but because of my set-up to enable getting both my satellite receiver, and Blu-ray player to use it, I need two. (My wife does not like going through the audio receiver, so I have it set so she uses the TV directly from the satellite box.)
But the way I connected everything, I do have both Darbee's being used when I switch inputs to my audio receiver, where the satellite receiver is also attached,which has the first Darbee, along with the Blu-ray and Roku. I then have a second Darbee between the audio receiver and the TV. Thus anything coming from the audio receiver passes through the Darbee - that one Darbee only for the Blu-ray and Roku, but both Darbee's when using the satellite receiver which is coming from an HDMI splitter.
I can't say it's worth buying a second just to have two inline. But I can say it makes a difference. Like another poster, I found the first Darbee on about 50 to 55% Game or HD, and the second at about 15-20% POP adds even a little more depth, by doing what the pop mode does, darkens/blackens shaded areas without losing lightness grading. Still no artifacts, even a little more sharpness and still not artificial looking. When I turn them both off and then on during Hawaii 5-0 it's the Wow factor. Again, just one Darbee really is sufficient, and produces nearly the same improvement. But I can't deny the second one adds just a little more.
No one can say if something is worth it monetarily for someone else. For me, the price of a Darbee is well, well, worth it. I know of no other device on the market, certainly in that price range that does what the Darbee does. The detail, down to the stubble on men adds alot to the enjoyment of watching TV/movies.