[AS2] setMask 2 times on same mc

Hi guys,

Suppose I have 3 mc’s : mc1, mc2 and mc3.

trying to do mc1.setMask(mc2) works fine.
then if I try to apply another mask on mc1, the last mask will work but the first one already applied will not.

So:

mc1.setMask(mc2);
mc1.setMask(mc3);

Only the last mask will show.
I tried to search for documentation on setMask but without any luck. Which may be the problem?

Thanks