You can set the default number of strands for all colors in the Pattern Properties (Stitch Options tab). You can set for a single color with the floss properties for that color.
For one thing, the pattern is just the pattern. The ONLY use for the count is for the estimate of materials. If you have a pattern that says 18 count, there is absolutely no reason you can't do 14 count or 11 count of that pattern without any changes to the pattern at all. Remember, all you need to know is each block is one stitch. Each stitch has a symbol that represents a color. That's all you need.
28 count over 2 is actually 14 count. If you want to make a pattern that represents your actual count, just use 14 count. In your profile, or instructions, you can list the material as 28 count evenweave done over 2. It's the same as 14 count Aida (except far easier to do fractionals! )