Willi Egger wrote:
In your code you try to typeset two A5 portrait beneeth each other on a A4 landscape, which is impossible. ... By the way if you want to typeset A5 on a A4 landscape without intending to make a booklet, then you can use setuparranging[2SIDE]
Thanks for the explanations. Now I understand what's going on, and the wiki page makes more sense too. I'll add to the wiki shortly.
A related problem is how to demagnify. Then you could have (1) a high-quality, final mode that generates A4 pages, and (2) a draft, papersaving mode that demagnifies them to A5 and combines them in pairs (a la 2SIDE) into A4. I see how to use modes to do everything except the demagnifying (Context seems, perhaps wisely, to ignore \magnification).
`Never underestimate the evil of which men of power are capable.' --Bertrand Russell, _War Crimes in Vietnam_, chapter 1.