Multiple engines on a shared tank

Thanks all!

what kell said

Summary: Use the fuel adapter part under parts -> the yellow utility parts icon. Then apply mirroring to the fuel adapter (not the engine attached to it)

Also, you should try your best to symmetrically place each new fuel adapter nozzle on the bottom of the fuel tank so your rocket's thrust will be balanced. I find that the easiest way to make the fuel adapters symmetrical with the bottom of the fuel tank is to attach the first adapter a good distance away from the fuel tank's single built-in engine connection point so your new engine's bell doesn't collide with the first engine and then left-click on the new fuel adapter part and then open the part mirroring tool in the left-hand menu that appears when you click the adapter, you can turn on part mirroring for the new fuel adapter part and clone it x2, x3, x4, x5, or x6 times. Then, you can right-click on the first engine to clone it and then attach it to the new fuel adapter parts, repeat this engine cloning for as many new nozzles as you've added with the part-mirroring tool. I hope this helps, let me know if you have any more questions, thanks! :)

You need to manually place a fuel adapter nozzle part and manually position it on the bottom of the fuel tank where you want the additional engine. The additional engines can be attached to the fuel adapter part. Best of luck! :)