I have given it considerable thought;
If you set 2 headsails, my experience has been;
1) needs to be about a 6'sprit, to give a reasonable gap to haul the genoa thru, otherwise you will be going forward quite a bit to 'help' the fouled sail.
I know folks with a 5' sprit that have to roll the sail up partway to get it to tack without hanging.
2) the old headstay on the stem needs to be attached lower on the mast to keep the gap open.They don't need to be parralel , a 4' gap at the top would do.
3) this means you need to add running backstays to support the pull from that stay , otherwise it can bow the mast to the point of failure .

If you just want to get more sail forward for light air, a shorter sprit will do , but I find it to be a rather halfassed affair that is a real pain to try and tack .