The first thing to note here is that Sand Veil doesn't actually increase a Pokemons evasion in sand, but rather decreases the opposing Pokemons accuracy. In the grand scheme of things, it's essentially the same thing as evasion, but it's important to note here because it shows that Sand Veil and Double Team can't really stack and raise evasiveness more, because they modify two separate things.
To answer your question though, when Sand Veil takes effect, the opposing Pokemons moves accuracy is multiplied by 4/5, and with 6 double teams, your Pokemons evasion becomes 1/3. This means that it in that scenario, any moves used against your Pokemon would only have a 4/15 chance in hitting, or roughly 26.7%.
Hope this helped!