Protoss sentry units have the Guardian Shield ability which:

Creates a range 4 aura that reduces incoming ranged damage to friendly units by 2. Lasts 15 seconds.

If a unit is sitting inside the guardian shield created by TWO different sentries, is the overall damage reduction still just 2, or is the damage stacking and first reduced by 2 for sentry shield 1, and then another 2 for sentry shield 2, for a total of 4 damaged mitigated?