They are both business owners, and choose to work together for the client's success. The truth is that by the time that most people do it, it's far too late. Don't wait till you're overwhelmed. Do it far sooner. This is very important clarification.
Most people think that time is when they can afford it - the truth probably is different.
Once you're overwhelmed, you have to play catch-up. The truth is that most can't afford NOT to have an assistant. In business, the most important thing to consider is opportunity costs. A terrific VA has more resources than you can imagine, and there's next to nothing she can't get handled for you. Certainly, most of them won't do things that are immoral or illegal, but past that, you're really only limited by your imagination and needs.