use one image background for flexible width of form buttons
To use sliding doors technique you need to consider the overlapping part. If your background color or image isn't fixed, then you'll have to deal with overlapping part of the round corners.
I didn't want to separate background image in left and right when i use sliding doors, so I tried to see if i can make one single complege background image and use CSS to offset a little
It requires two wrapping containers, both has the same background, one aligned to left, the other to right. The one that has its background background image aligned to right actually slightly offsets from its original position so the overlapping corners won't reveal the images below.