Megillah 30b - Torah Reading on Public Fasts

The gemara implies that if a Ta'anis would occur on Monday and we would be able to accommodate for reading both the parsha of the week and the reading for the Ta'anis by having the parsha of the week read by shacharis and the Ta'anis reading by mincha, we should do so. But since the Ta'anis for rain did not allow for reading by shacharis we forfeit the regular parsha of the week for the Ta'anis reading. Based on this, when one of the 4 fasts or Ta'anis esther falls on monday or thurs, why do we read vayechal both by shacharis and mincha? Shouldn't we read the regular reading by shacharis and vayechal just by mincha?