As the engine fires once every revolution (or 720° / 2 = 360° crankshaft angle), the two pistons run exactly in the same direction as well as position. That means the total vibration will be twice the magnitude of that generated by one cylinder. The direction of vibration is mostly upward / downward.
This is the worst engine configuration for refinement, therefore only the cheapest mini cars in the past employed it, such as Fiat 128, Today, I'm afraid there is probably no mass production car still using twin-cylinder engines, not even the smallest Japanese K-cars. Although the displacement of K-cars is 660 c.c. and is theoretically suitable to twin-cylinder motors, they employs 3-cylinder or even four-pots to avoid the severe vibration problems of twin-cylinder motors