Mary,

The lower halve of the mainsail is somewhat different in draft and twist profile when it is designed to sail with a jib. Or at least that is what you ideally want.

valtteri, is right. It is possible to tune a 2-up mainsail to sail singlehanded well, but not the other way around. In very competitive sailing I suspect that a custom made singlehander sail will be more competitive though.

Personally, I had my mainsail cut to suit my expected 150 kg combined 2-up crew weigth. I use this all the time when I sail singlehanded and it does surprise me how fast it is in that usage. It is a little more work to tune it right but I'm not killed on the course by other boats. I have too little experience with optimized singlehander sails to comments further on those.

But as stated above; I'm quite happy with the decision I took in 2003 to go for a 2-up mainsail and try to tune it down to 1-up usage. I'm using a relatively flexible alu mast, this may help.

Wouter