How do I group two nodes on the frontpage in a div?
The node template is not (usually) aware of the context in which it's been called; when you list nodes as teasers each node is themed individually, and has no awareness of the other nodes in the list. 

What you need to do is find out what template/function is responsible for outputting the list of nodes in question and override that in your own theme/module to add the extra divs.

The Theme Developer module might come in quite handy here, it provides an in-page way of isolating which theme functions have been responsible for rendering which areas of the page.