In my early MB years before finding the powerful GP60 engine, the rare Tanaka 47R racing engine and shift kits, I needed more speed. My 2.2hp Mitsubishi TLE43 was okayyy, but not a sprinter. But if one is good, then two Mitsubishis must be better!
And they were!
I installed a front-engine friction drive and a chain drive rear-engine on a 7-speed cruiser bike All worked well. Power galore, especially for climbing hills and punching thru stiff headwinds. Gas mileage was reasonable, and there was that AWESOME drone of two engines working effortlessly together. I ran the bike for 10 miles @ WOT without a whimper. Of course, there were drawbacks. Front end took a pounding, so I moved the engines to a single-speed girlie bike w/ROCKSHOX suspension fork. Another drawback was that the OEM tanks couldn't be filled to the max and didn't offer adequate cruising range. Refilling individual tanks were major pain. I had to literally run the bike up a tree to fill the front tank. I solved that by installing a common auxiliary tank on the girlie bike's slanting top tube. That caused problems for the front engine, whose carb sat above the tank's fuel level. I tried to solve the issue with a siphon bulb in the fuel line. That worked somewhat well. Then I installed a 12volt electric fuel pump made for a VW Bug engine. It was a major production to make sure each engine's main tanks were filled. Another disadvantage was the bike was very heavy @ 105lbs. The biggest negatives were the friction drives, and the bike ate thru many many tires. If I'd installed chain drives front and rear, it'd have worked a little better for me. Weight was a major factor in looking for a lighter, powerful MB. I found speed with the 4.5hp GP60 engine. When connected to friction drive, it literally took chunks of tread over every inch of the tire. Converting it to shift kit solved that issue, and I have not had to change a single tire in 2 years.
JMO, twin engines worked well enough, until I found REAL power in a single engine setup.