I have used 2 different methods to widen the handle bars. One is to use a small import car jack in-between the bars close to the top and "jack" em apart. This is the simplest method but not without risks. You depending on both bars moving the same amount, which is unlikely to yield perfect results. It makes "bad" bends look better but is no means perfect. The risks are you may crack the handle bar mount, end up with 1 of the 2 bent a little more than the other, and in extreme cases start twisting the bar close to the mount. You have to be careful. It's just something you have to be aware of.
The other method method requires the use of 1/2" rigid tubing bender. Works well but, have to do one bar at a time, may still crack the mount, but results can be as good as your bending skills are and home much effort and time you put into make them perfect.
It all depends on how close to perfection you're trying to attain, and your skill level at using a tubing bender?
For sure, don't even think about using a pipe wrench. Seen way too many with teeth marks on the bars.
Will post some pics tomorrow, and maybe this will give time for others to weigh in.