If you mix flat and semi-gloss you'll end up with something in between.Since the bases of flat and semi-gloss paints are entirely different I wouldn't recommend it.If you are dealing with a decent paint supplier,they should exchange your flat for semi-gloss unless it's a custom color you had mixed.In that case,I'd ask them what their paint will do when mixed.

what will be the result when mixing flat paint with satin paint?

Is the semi-gloss for trim and doors? We always use flat on stucco and some wood siding. How much did you buy? You can always use the flat as a base coat and then use semi-gloss as the top coat.You should find a way of using the flat as an undercoat. I would personally never use semi-gloss on stucco or siding of any kind. We use it for trim and doors only.