Unique over 2 columns I have 2 columns col1 and col2

INITIAL QUESTION:
is it possible to make a constraint to not be able to add the same email two times in this columns, only using indexes?

The Question has been edited to:
"Is it possible to define a constraint so as to be unable to add the same email twice in these columns for the same record, using only indexes?"

But it is my fault, I did not give enough details.
When I have a unique column. I can not repeat a entry in this column.
What I need is to expand this "unique" to a second column, so that any entry made in the first column, should not be possible entered to the second column at all.

I can do it using a external program by consequently adding the entries for this two columns in a array and always checking the array before entering something to this two columns. But the solution a joke, it is sub-optimal, because by extrapolation we do not need databases any more we can use arrays.

Is this possible using just mysql, or do I need "my array" ?
I guess you can use CHECK constraint

ALTER TABLE foo 
  ADD CONSTRAINT email1_is_not_email2_CHK
    CHECK (email1 &lt;&gt; email2) ;