what is the most effective way to maximize salary increase without being unappreciative, insulting, or disrespectful?

I work for a small to medium size business, promoted and trained to highest role in company apart from owner. My relationship with my boss isn’t strictly professional. Recently I assumed responsibilities of the owners second business on top of my current responsibilities. No one else is capable of doing this work but me, maybe the owner is capable, but he would need my help to learn it again. Quite frankly, I’m now doing work that calls for a new title.

I’m grateful for everything I have received/earned, I am well paid. Although, these new added responsibilities are not light. I’m capable of doing them and my current role combined, but doing it at the same salary seems like I’m doing 2 jobs for the price of 1. And before anyone questions why I accepted these new responsibilities without having this conversation first. These new responsibilities are essential to the operation, and must be done timely or else significant financial penalties start accumulating. I wouldn’t leave the business (or its owner) in a position where the sky is falling. Maybe a mistake on my part, this can be framed either way. Business is business, right? Regardless, it already happened.

My boss has certainly avoided having a real conversation regarding this, I’m certain he’s aware of what’s going on and doesn’t want to address it for financial reasons. Side note: this is why I am regretting assuming these responsibilities without having the salary negotiation conversation first. If he shared my goodwill, I wouldn’t have to bring up a salary negotiation. Learn from my mistakes people!

This conversation certainly needs to happen. Although my experience isn’t lengthy in terms of time spent doing it, I’m really good at what I do. And have gotten significant pay increases in a short period of time.

I haven’t done a great job previously negotiating my income. My question is, what is the most effective way to maximize salary increase without being unappreciative, insulting, or disrespectful?