Imagine, before you is a classic ICE (how many cylinder is not important). No matter how you deal with the inherent reactive forces and moments, they still break out and a lot of annoy the driver. And now imagine: we will dock another exact same motor with it, but we “twist” it in the opposite direction. In the "double" there are the same forces and moments, but opposite in sign - and our tandem is free from vibration completely! Of course, due to the imperfection of the production processes (in other words, the details are crooked!), Something will remain, but it is not so significant that a barely perceptible shiver can be neglected.
Let's be corrosive - look for the flaws of the scheme. But they are striking. There are twice as many parts - the manufacturing process is just as expensive. Further, the mass and dimensions will grow. Although this can be argued. Firstly, if the four cylinders are placed “square”, then such a layout in width will give odds to the row “four”. Secondly, thanks to the minimum vibrational load, engine parts (especially crankcase) can be extremely lightweight.
I dare to assume that it was such an argument that the young English engineer Edward Turner used when he seduced Jack Sangster, the owner of the Ariel motorcycle company, at the end of the 1920s to start making a motor tandem: put together two in-line “deuces” one after another, connected by gears and rotating in different directions, and so tightly press them against each other so that the valves of both blocks actuate one camshaft. The designer persuaded the businessman - an experimental 500 cc engine was soon ready. And it worked so "smoothly" that it was tested in the chassis of a 250 cc motorcycle. The production copy, dubbed Square Four ("Square Four"), made its debut at an exhibition in London in 1930.