If "Entwined Power" is used on Downpour Drenches the World and another Spirit: can lands with 2 of the other Spirit's Presence be treated as Downpour's Sacred Sites (and thus Wetlands) while the other Spirit is targeting?

If "Entwined Power" is used on Downpour Drenches the World and another Spirit: can lands with 2 of the other Spirit's Presence be treated as Downpour's Sacred Sites (and thus Wetlands) while the other Spirit is targeting?

No, only while Downpour is targeting. "Entwined Power" is "each Spirit may use the other's Presence while it is targeting", not "both Spirits' Presence belongs to both of them while either is targeting".
There is no formal definition of "using another Spirit's Presence"; informally consider it "treat it as yours for anything related to the act of targeting".