We have 6 window cleaners now, it's a big decision as to when to bite the bullet and employ, personnally I think the key is when you start falling behind with work.
If you fall behind then you have lost that money, you can't go and clean a house twice to catch up. Take someone on on a trial basis, either pay them wholly on what they do or pay them a basic plus a percentage of work done.
My guys are on a basic plus a 3 monthly bonus. Two guys working together will generally be able to do more than two guys working on their own, teams of three don't work.