Q:  Can you mix a flat latex paint with a semi gloss paint to achieve a satin paint?  If so, what would be the ratio of the mix?

A:  If the two paints are from the same manufacturer and are of the same type of resin, such as "exterior acrylic latex", you should be able to mix the two.  Only you can determine the ratio by experimentation. Make a note of the ratio when you perform the experiment. Start by adding small increments of the flat paint to the gloss paint until you are happy with the final finish.