There is a chap (active on these fora) who put two 20" dobs side-by-side, with mirrors made for the purpose by one of our premier opticians. I understand that the views through this 'binosaur' were so engrossing people needed to be prised off the eyepieces with some sort of lever, and reminded to breathe throughout the experience. These comments are anecdotal, because other than constant hyperbolic verbiage about the superiority of the views to, for example, a 25" f5, I've never actually had a chance to look through it...
I've got two 8' f6 mirrors fine ground with the same tool to the same (as near as I could get it) focal length to make a smaller version, but that's as far as I've got.
Same chap can present a number of well reasoned and articulate arguments about why the view is signficantly better with two eyeballs getting the full blast.
regards,
Andrew.