Seaming HardiBacker to Drywall
When I did my shower, I lined it up so the Hardi/drywall seam was about 1" before the edge of my shower tiles. Then I just covered this seam with mesh tape and thinset. So when I laid tile, it overlaid onto the drywall by about 1". I then used a caulk matched to the grout color run along the outside edge of the tile where it met drywall.

The tile edge is a good 5" outside of the tub, so I wasn't too concerned with water getting through the grout onto the drywall.

This gave me a nice clean edge and avoided having to try to get tape/mud smooth on the hardibacker. I've never had any expansion issues with the different materials.