Hi Iain,
A scaffold like that is a bit of a disaster waiting to happen. Is there a good reason you cannot have legs at the full width? You could calculate for that set up but the problem would come if say, six guys decided to sit together for their lunch break with their legs over outer side to watch the pretty girls go by.
Suddenly the best laid plans get screwed up instantly and people get hurt. Soooo, I would be exploring every possible avenue for an equal base before embarking on the riskier approach.