I don't think the word 'effective' is noun or widely accepted as noun. Maybe, in rarest case it could be but still I doubt.
Both are adjective and mean different things.
effective (adj) - producing the result that is wanted or intended; producing a successful result.
efficient (adj) - doing something well and thoroughly with no waste of time, money, or energy.
The meanings specified here are clear.
Now your case...
This has been considered as an effective technique for many years - the technique is effective because it proved the result what's wanted. In other words, its effects produced the results and that's how, it's effective.
The second one...
This has been considered as an efficient technique for many years - the technique itself is efficient. It's about being efficient as the technique here is. However, being efficient, you are effective* as well.
In other words, you are efficient to complete the task calculating 10 years' profit and loss of the company in an hour AND to prove yourself efficient, you must have effective technique! You may use some account software as that's an effective technique finish the task in said time.
Though both may go as an adjective to technique, it depends on the context what you want to say.