Restrictions for a specific situation in my DB

Hi,

I have a table with the following structure:

------------------------------------------------------------------------------

FIELD TYPE COMMENTS
------------------------------------------------------------------------------
id serial primary key
m1 integer id of record in another table
m2 integer id of record in another table

So, "m1" and "m2" are foreign keys.
I have read that it is a good practice to define an index for each foreign
key so "m1" and "m2" are also regular and independent indexes.

Now, the combination of "m1" and "m2" together cannot be duplicated. So for
example, the following entries

---------------------------------------------------------------------
id m1 m2 COMMENTS
---------------------------------------------------------------------
1 2 10
2 8 3
3 18 1
4 2 10 This is invalid.
5 13 8 This is invalid.
6 18 18 This is invalid.

Maybe the case for records 4 and 5 can be achieved by setting 2
restrictions:
Restriction 1: "m1 + m2" fields
Restriction 2: "m2 + m1" fields

But, is this a good approach?

Respectfully,
Jorge Maldonado