my friend and i had our girls about 2 weeks apart. I started watching her daughter when the girls were about 8 months old, and they couldn't find a suitable nanny after she had to go back to work.
it has been AWESOME! i only have my friend's daughter about 20 hours a week, but the girls have become such good friend's. they learn from each other, and it makes everything a lot less boring during the long, rainy season here in oregon
now i'm pregnant with #2, and even though my friend is going through a hard financial time right now, i'm continuing to watch her daughter for free.
i have been pretty tired, and it is so much easier for my daughter to have someone to play with when my energy is too low to chase and tickle her as much as she wants!
for nap times, we have a tandem-like arrangement. my daughter nurses, sitting on one half of my lap, and my friends daughter takes a bottle (of breastmilk-- her mom pumped exclusively through a more-than-six-month nursing strike... can you believe it?! i just had to throw that in there!) while sitting on the other half of my lap. when the girls fall asleep, i either hold them and doze myself, or read, depending on how tired i am!
this situation is wonderful for us.
it has gotten easier as the girls have gotten older (they are almost 17 months old now.)
during the hard times (when both girls are melting down) i put my daughter in the ergo carrier on my back (which she loves) and hold my friend's daughter until everything settles down.
i couldn't imagine doing this with someone who didn't have a very VERY close parenting style to mine, and i'm pretty sure that having a situation work out this well isn't the norm.