Restrictions for a specific situation in my DB

I have a table with the following structure:

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FIELD       TYPE                         COMMENTS

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    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 

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id             m1            m2           COMMENTS                   

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 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