is it possible to get to Beijing Ming Tombs from Mutianyu Great Wall?

The two places are in opposite directions and it is not convenient by bus with long duration. If you really want to go the two attractions in one day, the time must be tight. And you are not advised to do that.

The bus route from Mutianyu to Ming Tombs:
First take the bus No.936 at Mutainyu and get off at Niulan Mountain Daokou to transfer to bus No.947 and get off at Changping East Gate to transfer to bus No.314 and get off at Dingling Heshen Road. The ride may be take about 2 - 2.5 hours.