if you're doing absolute, use:

right: 0;
top: 100px; /* or whatever you want */

if you want it to float right, use position:relaive for #rightdiv

Don't use align="right" — that doesn't do anything besides invalidate your page.

If you want to actually float something, use #rightdiv { float:right; } — or float:left; if you want to, well, float it to the left.

Floats aren't as easy as relative, absolute or fixed positioning, but it's still something well worth looking into. In addition to learning about floats the second most important thing is to know that every time you float something you have to use display:inline; for IE. Otherwise the padding (or was it the margins) will double.

Clearing the floats is also a subject worth exploring.

I think this is what you're after:

CSS:
#wrapper {
position:relative;
width:100%;
height:100%;
}
#leftdiv{
position:absolute;
top:0;
left:0;
width:370px;
height:781px;
background:#eee;
}
#rightdiv{
position:absolute;
top:0;
right:0;
width:370px;
height:781px;
background:#ddd;
}

HTML:
<div id="wrapper">
<div id="leftdiv"></div>
<div id="rightdiv"></div>
</div>

But kari.patila is right about using floats instead of positioning elements like this. Try removing the 3 positioning properties altogether on both elements and apply floats instead (float:left, float:right etc.) and see what happens.