How to add CSS if element has more than one child?

How to add CSS if element has more than one child?

I have td tags and a several div inside td:
<td>
<div class='test'></div>
<div class='test'></div>
</td>
<td>
<div class='test'></div>
</td>
I want to add margin-bottom to div if there are more than one in the td.
How can I do this with the css?