A clinical study compares two drugs with binary success rates. If the true proportions are $p_1=0.6$ and $p_2=0.4$ with $n_1=n_2=100$, what is the asymptotic variance of the difference in estimated proportions $\hat{p}_1 - \hat{p}_2$ under the null hypothesis $H_0: p_1 - p_2 = 0$?