When dual wielding you should choose two weapons that have the same weapon speed. This is to ensure that the weapons swing at the same time all the time, thereby reducing the likelihood of delaying a swing while you time your combat arts.
It is much easier to time your spells or combat arts to one weapon speed than to two. Knowing that you only need to pause slightly once every 5 seconds is a lot easier than knowing you need to pause slightly at 3 seconds and also at 5 seconds. And imagine 3 rounds into battle when one weapon would swing at the 9th second and the other at the 10th second. That would not give you enough time between auto attacks to even use an instant combat art.
If your weapons have different speeds, you have to either pause your combat art chains more frequently, or just accept the occasional loss of an extra auto attack swing. Both choices result in an overall loss of DPS. Therefore, you are best off using two weapons that have the same speed.