When you have two speedlites in a softbox firing at the same time, let's say at 1/64 power, what is the total power output?
Doubling the number of lights at the same power doubles the output. Assuming both speedlights are the same to begin with, and if they're both set to ¹⁄₆₄th power, that'll be like setting just one of them to ¹⁄₃₂nd.
That's in terms of just the output, though. Since both flashes can't be in the same place, and therefore their relation to your subject not the same, your results will be a little different. If they are far apart or pointing in a different direction, they will be very different. In a large softbox, this is probably a good thing, as you can spread out the light more. If they're basically close together relative the subject, you don't need to worry about that.
However, this answer is as it is because you ask about fractional power. If you are looking at guide numbers and the amount of illumination, there's a little more you should know — see Does adding additional strobes cumulatively add to the flash power?. It doesn't matter if you have one flash at ¹⁄₃₂nd or two at ¹⁄₆₄th, but in any case, the guide number is proportional to the square of the power, so each halving or doubling is a √2 change in GN.
That is, if your flash is GN 64, at ¹⁄₆₄th power, it will be GN 8. Two flashes will give GN 11 (or, 11.314, more exactly, but it's convenient to round in the same way we do with aperture f-stops), and so will just one flash at ¹⁄₃₂nd.
If the arithmetic and power math make your head spin, there's a handy online GN calculator at the ScanTips site, where you can play with both power levels and combining multiple (equal) flashes. On the other hand, if you want to know more about the math, see What is the relationship between Guide Number and flash power level?
I also think you are after the math of “Guide Numbers”. The Guide Number is the timeworn method used to calculate the aperture setting when using flash. Once the Guide Number associated with a flash unit is known, we measure flash-to-subject distance and divide the Guide Number by the distance. Example: Guide Number = 80 and the distance =10 feet. We do the math 80 ÷ 10 = 8. We manually set the aperture at f/8.
When two flashes of equal specification are used together from the same position to get more light, the revised Guide Number is obtained by multiplying the Guide Number for one unit times 1.4. Thus if the Guide Number is 80, then 80 X 1.4 = 112. For thee flashes the multiplier is 1.7, for four flashes the multiplier is 2.
By the way, you can make exposure modifications to the Guide Numbers via a multiplier. To change exposure +1 stop the multiplier is 0.7 for +½ stop the multiplier is 0.84 for -1/2 stop the multiplier is 1.2 for -1 stop the multiplier 1.4
Hope this helps!