Can I block while using two weapons?

No there is not.

The only way to block/parry attacks, is to have either only one weapon, or a weapon and a shield, and then press (and hold) right mouse button.

Dual wield in game-land is always a tradeoff: you are doubling your offense in exchange for defense. So it could be seen as a balancing choice on Bethesda's part. Equally, it could have been forced on us by the people who play the game on consoles and their lack of buttons.

IRL, dual-wielding is done for the opposite reasons. You gain off-hand defense, but you'd not normally want to attack with it.

There is a mod that allows parrying (effectively blocking) while dual wielding. There are several options available on how it will handle using 2 keys to do 3 things.