It's been done..there was one with two 3.2 Sachs and one with two Q35s..
There are some larger ones, also an inline 3 made from 4.2 Sachs/Precision Eagles...Flown by Chip Hyde at Madera, went about 180 mph on all three, about 150 when one of the three came loose...Dennis Crooks had a P38 that went 195 mph using two 8.8 inlines made from Husky parts..The engines were coupled together by grinding a hex on each shaft a plugging them into a hex socket...The Q 35s had a different coupler and made a heavy engine..Dick Sizer from Minnesota coupled two Go Kart engines together, weighed about 18 lbs, made lots of noise but never ran well...Bill VanLeeuwen made a really nice 250cc inline, ran well, awesome sound, but was not competitive with the 289cc Herbrandson oppposed twins..
There's a website in Canada that advertises an engine called a Merlin, two 3.2 Sachs engines, about $2500.00..I had an inline twin made from two G62 engines, hex coupler, got looser each time it ran..Made it back into two singles....3W made a good inline also....
Most inlines are kinda heavy for their power output.....
The hex coupler seems to be the best way, it needs to be a very tight fit to stay together....