You should be able to use any weapon to block. Now, using a dagger to block a claymore shouldn't be all that effective...... but, using your longsword to block a dagger? Shouldn't matter what you have in your other hand.
A dagger is used most efficiently as a stabbing weapon, you won't be able to block that with a sword.
A sword however, is a slashing weapon that can be used to stab, but much slower and has a larger surface area - blocking it's motions with another sword is feasible, especially when slashing.