specify whitch index to use

hello,

I have two indexes on a table on cols col1 and col2, the table has ~10M
rows on pg v8.1.4

when I use "where col1 = val1" the query is fast and returns 0 rows
when I use "where col2 > val2" the query is slow and returns ~1M rows

in both cases the corresponding indexes are used.
when I use "where col1 = val1 and col2 > val2" the query is slow and
returns 0 rows using the index on col2

is there any way for postgres to use both indexes when relevant columns
are present in a where clause ?
is it possible to specify which index to use in a query ?

thanks,
razvan radu