First one should be: "Since you are busy, you don't need to come." Alternatively: You don't need to come since you are busy.
Second one is correct in one context, but is overly formal. As for whether you should use it or not, it would depend on meaning.
"As you treat me, so I shall treat you." Means I am going to do to you as you do to me. In other words: if you are friendly to me, I shall also be friendly to you. In that context your sentence works. Although it is, as I say, very formal.
Is that how you mean treat?