I have a table which has only two fields, id1 and i2. 

Both are ids from other tables and are int(10).

Should I have an index on this table?

Edit: What I'm thinking is that this table is made up of two simple data types... would MySQL query the non-indexed data less efficiently than the indexed data? 

The table in question will have more reads than writes and wouldn't be queried very often so an index would be fine to have.