Hoping this is the best area to ask this, mostly in the DIY and hobby world I've seen 2 engines coupled together to make a twin engine some so this for the zen of doing, but others do it to align the pistons I've noticed particularly in RC aircraft rather than buy a twin engine where the pistons operate at exactly the same time and I supposed matched this Introduces a reciprocating vibration that causes issues.
I've been hearing if you take 2 two stroke engines and couple them together with 1 at TDC and the other at BDC they cancel each other out and a guy on YouTube showed a video of one with a glass of water next to it barely moving.
What is the name for such an arrangement? Is there any disadvantage of doing this? Links to more information? I shall search for YouTube video as described above.