my girls are 15mths apart and from day 1 DD2 slept in the lounge with DD2 running around making noise, if we weren't in the room i left the radio on so there was constant noise.
Now she is in her own room i find she does wake a little more if DD2 is being noisy outside, am thinking of getting a radio to put her in room so there is constant background noise and she is less likely to wake.

My other issue i think is DD2 is now nearly 6mths and very curious of things so is more likely to wake to investigate.

I hope someone has the perfect answer for you, DD2 pretty much does 1 sleep cycle aswell.. damn kids.