I have two and use them both at the same time, I have to say its made a big difference for me.
I don't always beat the lamp but majority of the time I do (second colour coat, art etc) so it helps out with that. I find my clients are less likely to mess up their nails, get fluff on them etc now I have 2 lamps. The positioning of my clients is now easier for me too, I used to have the one lamp to the clients left and they'd always twist their bodies towards it, even if I moved it which could be uncomfortable for them and me. It's also helped with my bigger and older clients who find it a lot more comfortable. Other than that I now have the back up of the second lamp should one break. Here's a pic which shows my set up