Double ost mortars are hard. However, in the early game this is quite an investment, which means either less grenadiers or less support weapons to protect the mortars. Most of the time it means less grenadiers, which in turn means your riflemen can take advantage of well executed flanks.
Most of the time the mg screens the mortars. If you smoke the mg with 1 riflemen and use either an RE or that riflemen as bait, you can move in 2 other riflemen once the mg is either blind and stationary, or moving to another position. This gives you time to move in on the mortars and wipe them. There are too few grenadiers to protect if he went for double mortar early in the game.
If this fails, I suggest an m20 dive and try to kill one of the mortars with the pintle mg and the other by hopping out.