Attaching an event to multiple elements at one go
The jQuery add method is what you want:
Adds more elements, matched by the given expression, to the set of matched elements
var a = $("#a");
var b = $("#b");
var combined = a.add(b)
Don't forget either that jQuery selectors support the CSS comma syntax. If you need to combine two arbitrary collections, everyone else's suggestions are on the mark, but if it's as simple as doing something to elements with IDs a and b, use $('#a,#b').