planner doesn't use multicolumn index

We've a table with about 8 million rows, and we need to get rows by the value of two of its fields( the type of the fields are int2 and int4, the where condition is v.g. partido=99 and partida=123). We created a multicolumn index on that fields but the planner doesn't use it, it still use a seqscan. That fields are primary key of the table and we clusterded the table based on that index, but it still doesn't work. We also set the enviroment variable enable_seqscan to false and nathing happends. The only way the planner use it is in querys that order by the expression of the index.

Any idea?

thanks.

Adrián