Just to offer another opinion...
We did buy another cot. We decided that DS was just not mature enough to cope with a big bed. He's little-ish anyway.
We also tried TT, but it was too much for me as I had SJD and was struggling to deal with the bending down etc. We delayed TT until after DS2 was born and it went perfectly smoothly. I use flats/prefolds for the newborn anyway and DS1 didn't need so many nappies by then, so i didn't even buy anymore MCNs. (it was summer too mind you - that helped!)
I have a Phil and teds with the double kit. DS1 loved to walk and i had many baby carriers too, but i have still found the double pram invaluable! We sold the old one and bought another.
I feel that the boys could have easily shared a room, if we had needed them to. DS2 did not move out of our room until about 6 months anyway, and when he has a big cry he wakes ds1 up anyway - one wall between them makes little difference.

(BTW - there is 21 months between my boys)