The maximum percent of cross sectional area fill when installing more than 2 conductors in a raceway is

Is on page 70-671 of the 2008 edition of the National Electric Code.

The 2008 NEC gives a 31% fill for two conductors in a conduit.

However, you still have to make sure you are using the proper size and type conduit for what you need. And you also need to know the size of the conductors you will be using.

For example if you wanted to put two 4/0 THHN conductors into a trade size 1/2 EMT conduit you are not allowed to do so because they will not fit. You would have to use trade size 1 1/2 EMT conduit.