Just seen this one. Don't know if it will work but try putting A5 into Page Set Up and fiddling from there. Alternatively, buy Blue Squirrel's ClickBook which will do just what you want - I have used it for years to produce booklets.
I am not an Office whizz kid, although I use it but have never managed to make separate Header footers...it would be useful for assignments (when I use a non automatic method for each page) and other reasons I guess there is maybe no answer because nobody knows?
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