If one of the stock twin is leaking oil. Does it mean only turbo need to be replaced or turbo and engine both need to be replaced? Will it affect the engine at all? Anything else oil leaking might be affecting other than turbo itself?

 

If just the turbo is leaking than the engine is just fine. All you will need to do is have the turbo rebuilt and make sure to replace any gaskets. Oil leaks from the turbo won't affect anything else unless it catches on fire or something