Mixing paints of different sheen to get a specific sheen can be done but it will be hard to match in the future.
The desired sheen is obtained by amount of binder and pigment in the paint, and mixing paints with different sheen does allow you to shift the sheen without compromising the chemical properties of the paint.
When repaired patches are later re-painted with a mismatched sheen you get what is called "flashing": depending on how the light falls in the room you'll see dull/flat or wet/shiny looking patches.
It is better to apply the wrong sheen as the first few coats and finish with the desired sheen. Then if you ever need to patch up in the future with some new paint you know it will match perfectly.
For mixing sheens you'd have to experiment with ratios, and you might end up needing much more of one paint than the other, making the paint purchase inefficient for small quantities and when on a budget.
Colour perception and sheen perception depend on each other, and you'd have to let the paint dry for every experiment before deciding on a ratio. This can be cumbersome if you are critical.