#contain {position:relative;}
#actual-black {background-color:black}

.overlap {
    position:absolute;top:1em;left:0;
    background-color:black; opacity:0.2;
}

<div id="contain">
<div id="actual-black">The background is opaque black</div>
<div class="overlap">This is transparent black</div>
<div class="overlap">This is transparent black</div>
<div class="overlap">This is transparent black</div>
<div class="overlap">This is transparent black</div>
</div>​

D1's opacity = 0.1
and
D2's opacity = 0.2
their overlap opacity = 0.3. (can it be?)