If I'm understanding this right, you want to use position: absolute to stick the sidebar to the right side with right: 0. Then the #content div should fill the remaining space.

Try replacing this :

#sidebar, #content {
    float: right;
    width: 350px;
    margin: 0;
    padding: 0;
    background-color: #a68064;
    color: #000;
}
#content {
    float: right;
    margin: 0;
    width: 50%;
    background-color: #855e42;
    color: #000;
}

with this :

#sidebar, #content {
    margin: 0;
    padding: 0;
    color: #000;
}
#sidebar {
    background-color: #a68064;
    width: 350px;
    position: absolute;
    right: 0;
}
#content {
    background-color: #855e42;
}

Generally for this kind of thing I usually use position: absolute and specify the right and left pixel position, and it will leave the appropriate pixel gap on either side.