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Do you have a Bahtinov mask you can use? This will tell you if you can actually achieve focus or whether there is an optical issue at play. Bahtinov Grabber is a great utility to use with the mask as it quantifies how far (in microns) the sensor is from the focal plane, and will report when the sensor is within the Critical Focus Zone for your particular optical setup.
If you're just going off FWHM numbers that wont give you the full picture, especially if there is variable seeing or an optical issue. If there is a fundamental problem with the scope, then even if you've supposedly minimised FWHM you may not actually be in focus.
My suggestion is to set up both of your scopes, and then focus each one in turn with a bahtinov mask. Then take a few images and use Dynamic PSF in PI as Ray suggested and compare the results (once you've converted the results to arcsecs) for each scope. That might help you to determine if the issue is with your focus procedure, or with seeing, or with the optics of the DED.
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