We took one side off our son's cot a few weeks ago at about 2 1/2. Both sides come off to convert it to a toddler bed but we figured it looks more like his cot this way and the transition was more gentle.

He has handled the change beautifully and generally still stays in bed even if he's struggling to get to sleep- just calls out for us. But he getsup in the morning and comes in to wake me up which is lovely I'm glad we didn't try younger, I think it may have been more work and wouldn't have been ready. Personally if I had children closer together I'd get a second cot, or at least a cradle to give you more time to have the older one in their cot.