I'm not sure how well this would work in reality, for starters most dual SIM phones only provide 3G service onone of the SIMs, you have to physically swap then

I'm not sure how well this would work in reality, for starters most dual SIM phones only provide 3G service onone of the SIMs, you have to physically swap then over if you want it on the other. Such an app would also need to keep track of the credit on each SIM, bearing in mind there is no standard way to do so among all operators. It would also need to deal with differences between the phones reported data usage and the operators measured.