Hi elerose,
Look at your pattern instructions and see what they say. BUT when you are working with higher count fabrics, such as 25 or 28 counts or higher, YOU need to make the decision as to whether you want to use 1 or 2 threads over each square. My best suggestion is; on a piece of high count fabric you want to do a project on, stitch over one square with 2 threads of floss for about 6- 10 squares across and 2-3 rows down and see how you like the results.
BELOW this, stitch over one square using ONLY 1 strand of floss like you did using the 2 strands of floss and then compare the two examples. Which one do YOU like the best?

I started a pattern on 28 count fabric that suggested two strands of floss (as the instructions said) and after 2 rows it looked and felt like a carpet AND I was having any awful time getting my needle through the squares that were already done in the row above. I "frogged" the rows out and started again with going over 1 with just one strand of floss and was happy with the floss coverage on the fabric.
Many of HAED's patterns have instructions on using 25-count fabric OVER ONE in their instructions. I didn't like the coverage on 25-count so I changed the fabric to a 28-count (over 1).
Hope this helps..

That helps a lot! Thanks