How can I replace the top two wood panels in my front door with glass?
I'm assuming it's a wood door with rectangular frame and panel construction.

Often in cheaper doors, the wood panels are simply held in by trim on one (or both) side(s) - usually the interior side. If that's the case, you can simply (carefully) pry the trim out, remove the panel, and insert glass in it's place. You'd then want to seal the exterior side with a little silicone, and reattach the trim to hold the glass in place. 

If it's a higher quality door, the panels are housed in grooves within the frame, and it's assembled in one step. In this case, you would have to cut away part of the frame in order to remove the panel. The simplest method I can think of would be to use a router and a fence to remove the material. Set your fence so that the router cuts away about a 1/4 inch of the frame material on the inside. If that doesn't expose the panel completely, slowly adjust your fence so your router moves closer to the outside until the panel is exposed. You'll likely have to use a chisel to square the corners of your cut. Then remove the panel, replace with glass, and install trim to lock the glass into place. Paint/stain to match.

If you have the tools, it is actually pretty easy to make a double pane window. You would simply need 2 panes of glass of the same size. You would insert the first as normal, then use thin strips of wood, I'd say approximately 1/8 inch square to offset the second pane. Attach these strips tightly against the first pane,locking it in place like you would with trim and lay the second pane on top. Then finish with trim.

Edit: One note I just considered. Often frame and panel looking doors are not actually constructed as such. Some times there is a solid plywood core and the faux frame and panel is simply laminated on top. In that case, I would look into getting a new door.