My late friend Bill VanLeeuwen built a 250cc inline twin for racing...The hardest part is making a coupling strong enough...He got the coupling strong enough after many attempts, but the engine was not competetive with the opposed twins...
There are a few inline 3 cylinder engines around, made a few years ago..One is currently being used is a scale warbird...The engines are coupled together by grinding a hex on the end of the crank and using a form of hex socket to plug into...It stays together for about 3 flights before having to be re done...I have some crank parts here to check for straightness and put back together....
I had an inline twin G62 a few years ago, built by a customer in Colorado...Worked pretty good, but the coupling got looser every time it ran...Finally took it apart and sold it as 2 separate G62s....
The best inline twins were/are made by Challenge Tool, called Husky Challengers...I think the one in the Fastrat porting paragraph above is one of these...NO problems with coupling, don't know what it looks like, but the rest of the engine is museum quality.....