Engines have to be
absolutely identical.
It used to be (maybe still is) that karts running twin engines had engines that were purchased at same time. Some kart engine manufacturers actually sold them in pairs for the reason that certain key specs change over time due to tooling wear and other controllable/uncontrollable factors. No two engines are alike even under the best of conditions.
Someone in another thread posted a YouTube video with twin chainsaw engines and, if you listen to engines, are clearly
not synchronized. It does work and works well but one engine is dragging the other one down slightly and I would almost bet that its making the builder crazy
with adjustment tweaking of carbs.
If you are new to this hobby then I would only repeat what I and others have said.
Start with one engine. You have to crawl before you can walk.
Single engine kart kits are reasonably priced and fairly easy to assemble.