We did day sleeps for a week prior to trying night sleeps with DD2. With DD1, she slept better in a bed than a cot, but we were in denial that a 16 month old would climb out of a cot, so we put a mattress next to the cot and would lay on it next to her... she started sleeping on the single mattress until we bought her the junior bed.

Personally, if we were going through it again, I would keep them in the cots as long as I could. As long as they sleep well, don't go changing... if they don't sleep well, try whatever works best for your family!