My younger sister and I had to share a bed growing up... It really sucked! lol.
My sister wasn't fully potty trained though... and waking up in her puddle sucked really bad.
Once we got over that issue, it still didn't work too well, because I am a very light sleeper, and she is a very deep, active one... So she would wake me up several times a night.
It was also pretty miserable when one of us was sick.
Although... I do know several kids who LOVE sharing beds, and sleep better together than they do alone. This is another situation where it depends on the individual child.
IF I was you... I would buy two twin beds when DD2 is old enough. IF they want to share, you can push them together and use full-size sheets to combine them into one big bed... but if it starts to be an issue, or just doesn't work, have the option to separate them. :)