There were two sisters at my school who were 8 and a bit months apart, but i think that the 2nd was a couple of months premature. My two are 13 months apart.
in your second question, I guess it would be possible, but the probability would be small as you would be relying on a women releasing two eggs rather than one, having sex two partners in a short space of time and having each egg fertilised by the sperm of each man - statistically unlikely, but unless someone with greater medical knowledge than me knows a reason why not, i think that it would be medically possible.