Does "not so fast" stop all damage of Falling Star? Even if split targets?

Yes, Not So Fast can stop all damage from Falling Star, regardless of whether you target one unit twice or two different units.

How the interaction works:

1. Targeting: When Falling Star is played, its controller must declare all targets for both instances of damage simultaneously.
2. Reaction: If Falling Star targets at least one of your units or gear, you may play Not So Fast while Falling Star is on the chain.
3. Resolution: Because Not So Fast counters the entire spell, Falling Star is removed from the chain and does nothing. Since the spell is countered before it resolves, neither instance of damage is dealt.

Key Rules:

• Countering Spells: According to Rule 425.1.a, a countered spell does nothing and is cleared from the chain.
• Targeting: As confirmed by RiftJudge FAQ #8503, Falling Star targets units. Because it chooses targets, it is a valid target for Not So Fast, which states it can "Counter an enemy spell or ability that chooses a friendly unit or gear."
• Full Effect: Countering a spell stops the entire card's effect. It does not matter if the damage is split between two units or focused on one; the entire spell is countered, preventing all damage instances.

Note: This interaction assumes that at least one of the units targeted by the opponent's Falling Star is a unit or gear you control. If Falling Star only targets your opponent's own units, you cannot use Not So Fast to counter it.