We had ours together up until around 8 months, when there were too many wake-ups overnight because someone was getting kicked in the head. They slept in their own cribs (and with us when they wanted to) up until they rebelled against the cribs around their second birthday. Now they sleep in our old queen size bed in their room, and we have a king. Best move we ever made. They still often end up in bed with us, but at least now there is space!
I have never heard anyone say it wasn't safe for them to sleep together, though. Only that most people separate them when they become mobile, which I understood when it happened to us!