Essentially depending on your altitude, separation in the same direction could be 2000 feet or 4000 feet.
In opposite directions its 2000ft or 1000ft again depending on your altitude.
So this sight is common, very common, especially over the atlantic.
In the hold at ORD or LHR is good fun too! (assuming your not nervous about flying of course!).
If you want to see something to really shake you, get a window seat in the middle of the 777 wing and watch the wing flex during take off and climb!!
[This message has been edited by Icarus (edited 01 February 2001).]