The biggest problem with a "two cylinder" crankshaft is that there is a dead spot in the torque curve whenever either "piston" is at TDC or BDC.
When you have your feet directly on the pedals of a conventional circular crank, you actually have the ability to apply force over greater than a 180° arc — by a combination of flexing your ankles and pushing forward/backward with your thigh muscles — which eliminates the dead spot.
If you try to use a mechanical connecting rod with a crankshaft, you need some other way to eliminate the dead spot. This typically involves using more than two pistons, or by incorporating a large flywheel into the mechanism. None of this is practical or desirable on a bicycle.
I have seen toy vehicles for small children that use such a mechanism. but invariably, they need to put their feet down on the ground from time to time to push themselves out of the dead spot.