Ok, so I have determined part of the problem. Superfish IS rendering the second UL - it's just rendering it directly under the first. Here is how I've got the menu coded right now:
<ul class="sf-menu"> <li> <a href="#">Top Nav Item</a> <ul> <li>Sub Nav 1</li> </ul> <ul> <li><span>Sub Nav 2</span></li> <li><a href="#">Products</a></li> <li><a href="#">Products</a></li> </ul> </li> <li> <a href="#">Top Nav Item 2</a> </li> </ul>