our dts are 17 months, and have been in nursery full time since 12 months. apparently at nursery they can be quite naughty. at home they tend to encourage each other, so this is entirely believable! when we had one home for a day, the other was apparently much quieter and better behaved, so we thought we would experiment with trying to keep one or other home for the odd day when possible. dt2 seems ok, but dt1 really misses her brother. yesterday she was upset all afternoon, and kept repeating his name.
so, the question, is it wise to try and separate them to give them a chance to learn to be independent, and learn about behaving better? dp has always been keen on doing this, but I tend to think there's plenty of time for them to learn to be individual as they grow up.