Approximating Multiple Opponents

[bigchips]

If I am facing two opponents, and my hand will beat opponent #1's likely hands 50% of the time, and opponent #2's likely hands 70% of the time, what is the equation for approximating how often my hand will beat both opponent #1 and #2?

If I assume two opponents at 50%, I can deduce that I will win 33% of the time. I *guess* I arrive at that approximation by: (.5 + .5)/3 = 33%. So by way of this equation my anwer would be (.5 + .7)/3 = 40%. Is this answer a good approximation?

I realize that the way to definitively answer the question is to have pokerstove face off against two opponents. But if time is of the essence, is the equation above a good way to quickly and accurately approximate my win% vs multiple opponents or is there a better way?