I installed durarock for a new shower wall, think I was tired by the time I got to the last piece. There is a difference of about 3/8" between 2 sheets. I tried to feather it out with thinset when I taped all the seams, but I don't know if it will work long term. How much thinset can you use to even out seams between 2 sheets of backerboard? Wife spent a TON of money on the tile so I only want to do it once. Can I continue to build up and feather the edges out with thinset, should I try to stucco it, or should I rip it out and redo it?

Time to take it out and start over, hopefully you used screws. Use shims behind the cement board and screws every 8 " or so in spacing.

I have been in the business over 20 yrs and have seen some really bad "patchs with cement behind tile" work.. the most I have ever been able to cover is < 1/4 inch and still have a nice flat surface for tile. Of course with patience and practice you can build up the cement to a very nice smooth FLAT surface, but not everybody has that much time or patience.

Bummer. So if I was to try and build it up and smooth it out, I should use cement rather than the thinset, though correct?

Well, I ripped the bad sheets of cement board out. While I had it opened up, I decided I didn't like the way I framed it, so I completely reframed that wall. Tiled that wall last week and came out beautiful. Thanks for the push in the right direction.