Is it possible? If not, is there a way to do it with jQuery?
Unfortunately it's impossible.. Disregard this answer and look at spliters answer below.
If it were to be possible, it would look something like...
ul li:last-child+li {...}
But this doesn't work, because + will select the immediate sibling after last-child (which is nothing, of course). There is no immediate previous selector.
There are different ways of achieving this with jQuery, the most performant would be...
var lastItems = $("#list li");
lastItems.slice(lastItems.length - 2).addClass("whatever");
http://jsfiddle.net/qkzdJ/