First of all, half cover does not grant advantage on Dexterity saving throws. It grants a +2 bonus to the roll.
A target with half cover has a +2 bonus to AC and Dexterity saving throws.
The creature in half cover will get a +2 bonus to their Dexterity saving throw against all spells that require a Dexterity saving throw and do not ignore cover.
A creature is in half cover when:
A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.
Note that they only receive the benefits of cover when the cover is directly between them and the creature/spell they want cover from.
A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
This means that yes, the second creature in your fireball lineup will have half cover, provided the first creature obscures half of the second creature.
Fireball does not explicitly ignore cover. However, according to Jeremy Crawford, it can ignore cover.
Your cover is foiled if an effect spreads around it and reaches you.
-- Sage Advice
Because fireball does reach around corners, it can ignore cover. However because it does not explicitly ignore cover as below does, it is up to the DM to determine if a specific fireball ignores cover.
For an example of the wording on a spell that does explicitly ignore cover, see Sacred Flame:
Flame-like radiance descends on a creature that you can see within range. The target must succeed on a Dexterity saving throw or take 1d8 radiant damage. The target gains no benefit from cover for this saving throw.