Better way to check more than 1 value NOT IN (...)

Hi,
I've got to check if 2 values are not in a set that is dynamically 
calculated by a function.

The query looks like

select  some_id, ....
from .....
where   10  is not in ( select x from my_function (some_id))
and   20  is not in ( select x from my_function (some_id))

Is there a efficiency issue with calculating the set twice ?
Can I somehow check both (or more) values at once?