No way man. The whole bong would turn red and get hot before you started to liquify the glass. Even if you could melt the glass, they're not just gonna stick together. The two peiced would have to be pressed together. You'd basically be having to make a new bong.
I've allways just used super glue to fix my shit, but I'm sure there's a better adhesive out there. Maybe someone who works with glass could fix it, but I doubt you'd be able to do it with basic household tools.

I'm interested to see if I'm wrong though. If anyone else has had better luck, then speak up.