The advantage of twin headsails when downwind is that :

You dont have the sail chafe
If wind gets up, it is very easy to reduce the twins (even taking one,and then rolling both together is possible)
On a half boat, downwind you suffer from pendulum effect, it can get so bad that the boom ends up in the water. The foresails are normally shapped so that this is not a problem.
The twin sails are a much better balance, and more forgiving of apparent wind changes. In some boats, you can achieve a balance with them that assists the windvane in keeping course.

Can you not get a new foil for your existing system rather than pay out that much?