You may achieve 50% scaling with two 8600GT vs one 8600GT. But as you can see even if we consider 100% scaling for fun (not going to happen), a single 8800GT is still ahead.
At current prices, an 8600GT is a good card, close but not quite as good as the somewhat cheaper HD2600XT. But two 8600GT is not the best for the money. It easily puts you above the cost of an 8800GS or even 9600GT, which are both better solutions for the money.