Yeah going through an area with detector x after detector y, usually isn't a very unbiased test. As said already you can often go back through with the same detector, not even changing settings necessarily, and pull out more stuff. Honestly there might not be a truly perfect format for a "x vs y on the same site" type of test, but something somewhat close would be:
Go through site with detector x, mark but don't dig good signals, using some kind of markers, preferably numbered. Go through with detector y, mark good signals with the same markers, but don't put a second marker if there's already one there from detector x. That done, go back with both detectors, a shovel, and a piece of paper with a table written out on it. For each marker, go over it with both detectors, record how each responds, how clear the signal is, the numbers, etc. then dig the signal, ideally record what it is and the depth, and rank (scale of 0-2 probably: 0 is it doesn't hit it, 1 is it doesn't hit it well or hits it but ID'ed it wrong, 2 is it hits it well, and IDs it correctly) how each detector handled that signal. Do that for every marker, and whichever detector got more total points in the numbered ranking, did better. Doing tests with more than 2 detectors would work largely the same, and importantly, all three/four/whatever, have to be tested on the same patch of ground at the same time. This test doesn't work as a valid comparison for "x vs z" if you do "x vs y at site 1" then "y vs z at site 2" nor is it valid if you test "x vs y at site 1" then "z at site 1 after the xy test". You can only fairly compare three detectors by "x vs y vs z at site 1 at the same time" and so on.
This should work, and be simpler, with test beds, as then you can go over with x, y and z at different times, and don't need to dig as you know what is buried.
I put a decent bit of thought into trying to make that procedure as fair and unbiased as possible but I'd love to hear any suggestions if there's something wrong with it. The main issue I see with it is it's a very painful and high effort procedure to follow. If/when I get a second detector, I'll do that exact procedure and put the results here on the forums.