Funny you should ask, i've just at the point you are but have someone starting tomorrow. I think that your thinking is faulty.
You plus someone else does not have to do double work.You only pay for one van and the equipment on it, so if you consider this a large investment you are not using it very well with just one person.
Or put it another way, if you had two vans and did twice the work would this really beat one van plus help doing 1.5 times the work?
In my case i get massively, uncatchably behind after holidays and have to fight for weeks to catch up.Then another holiday and then the same mess. I hoping catch up with this help and then, when i have some time go and find more work like i used too.
In my early days i would often start a day with no or very little work planned, and then, following a phone call, or an odd thought to try an area return with my pockets stuffed with cash.
In all honesty i haven't got enough for me and this guy if we are hitting fours everyday, but perhaps i will have the odd saturday off, or i will take on some insides which i tend to avoid, or i will go to a new area.
At the minute all i do is work, and there has to be a better way, even if it does mean earning less money.