case structure problem with default 0, 1, 2

Good afternoon,

I have a question about the case structure. If you use 3 cases, default 0, 1 and 2, like in the following VI.

How can you fix that the sum is never 3? Because when you turn on the switch 1 and 2 simultaneously it gives the sum 3 and the case will go in the default stand.

Thanks for the help.