No, they do not override each other. Each function is executed.
You could of course check this easily yourself: http://jsfiddle.net/6jgGt/
Or understand from the jQuery code itself:
Line 255 is the ready function where the jQuery.bindReady(); is called which among other things initialises the readyList object on line 429 with readyList = jQuery._Deferred();
And once it's a deferred object the function passed in is appended with readyList.done( fn ); and we can see in the done method on line 41 that the element is added to an array with callbacks.push( elem ); so each one is saved separately...
