Granarchist, our DGS also did very well on his balance bike, zooming along with his feet off the ground. He also had a bike (from FaceBay) with stabilisers on it which he would ride around on. One day last summer, when he was 3 years and five months old we asked if he wanted the stabilisers taken off, we did so and that was it, he was off, no problem at all. I think balance bikes are great for giving them confidence. His current bike is a 12" and quite heavy. For his birthday in March there is a brand new bike waiting for him, 14" this time and at the moment we have left the stabilisers on for him to get used to it or he may want them removed straightaway. We got it from Halfords in the sale, £59 I think. It will be his first properly new bike.
How many pair of twins have you met?
