converting AC3-2 to AC3-5.1

Well there is no way you are going to get true 5.1 for a 2 channel source. There are I believe several methods on doom9 if you don't like the sound of that one.
There´s no possible way to get true 5.1 out from a 2 channel stereo source, all you could get is emulation encoded in a 5.1 format, the only way to end with a true 5.1 is having the original separate tracks before mix and remixe them.