When both light up it just means either both are available or both have had the buff fall off. From my understanding prioritize the attack buff skill before the haste skill. What has helped a bit for min/maxing is also kind of splitting them between left and right to match the job gauge. However, that takes wading through the atrocious ability window to figure out. With the left/right dynamic set you can then just know which is what buff or what position by relating to left or right, rather than the ability icon. I struggle with ability icons on every class, so I've had to develop hotbar setups that rely on position rather icons for skill identification.