Which one of the two is correct?
Doing something may not only be a personal decision, but also a business decision
Doing something may not be only a personal decision, but also a business decision
If we talk about the scenario which you've given above, first one sounds correct to me. As it is not only but also kind of a sentence. I hope you are clear with what not only but also is. And of course, if it's just about may not only be and may not be only, both are correct. Depending on which one sounds good to you, you can choose one over the other. I would always go with may not only be.