Putting two wall-warts in series?
To answer your question: if the wall warts are not grounded and are isolated from the mains then there should be no problem connecting them in series. I even connected a couple of PC power supplies in series, just made sure to connect them to ungrounded outlets.

Now, will ~30W be enough to heat the wire? Also, you should measure the resistance of the wire and find out what voltage and current it needs, because it may be that it has lower resistance so you would need to connect the wall warts in parallel to get 15V and 2A, but you shouldn't do it, since connecting two power supplies in parallel can lead to problems.