I would buy a second cot--no question. I do think it depends a great deal on your LO's temperament and there are a few LOs who successful transition out of a crib at 18 months, but they are very much the minority. Your LO is going to have a huge adjustment and you are going to have a little baby giving you sleepless nights, the last thing you need is a 19 month old with a sudden sibling-inspired sleep regression who can get up and run around her room.
The general recommendation that I read, and what I think Tracy says in BWSAYP is not to transition out of the crib until age two. My DS was 2.5 when my DD was born and we tried transitioning him to a crib just after age 2 and it was a disaster. He's spirited and was never a great napper and if we'd persisted with the transition we have had an OT monster on our hands who in all likelihood would have dropped his nap right then. As it was, we went back to the crib, bought a second crib for DD so there was no pressure. He had a really difficult time adjusting to his sister in general and I'm very glad we didn't push him out of his comfort zone sooner. Eventually he initiated the transition when he was a few months past age three.
I did feel a bit silly buying a second crib especially as he was over 2, but with his personality it was definitely the right choice. Plus, it's worked out well because I accidentally got pregnant and #3 will arrive when DD is just 23 months. With her personality, I think she would be more likely to handle an early transition relatively well, but she sleeps well in her crib and I'd rather stick with what works than push her--so glad we still have the 2 cribs from last time.
