Moving Layers And Control Speed

I have to move 2 layers (one from up to center and one from bottom to
center) and I found this way:

<div id="up" style="position:absolute;left:10px;top:-20px">Up</div>
<div id="down" style="position:absolute;left:10px;top:200px">Down</div>

function movediv(){
var u = document.getElementById("up")
var d = document.getElementById("down")
u.style.top = "100px";
d.style.top = "100px";
}

It doesn' work...because:

1) They don't move at "the same time"...I would like to obtain
simultaneous move.

2) I can't control speed so moving is invisible!