If we can only cast 1 of the same-named spell per turn why run 2 of each? effectively you need to pick a skink ball and ignore the other?
It's not about spells, you usually run 2 of each when you have 2x 40 skinks, so you can give them each serpent staff and star-stone staff, which are once per model, and the 1 of the same name spell doesn't count.
i tend to run 2 starpriest so i have more options for venom and redundancy. also i like to have celestial harmony as 1 spell and hand of glory as a second
The way I play double Starpriest+double Priest is: Every Starpriest and Priest babysit a unit of 40 Skinks, meaning giving all his buffs. One Starpriest goes with Celestial Harmony and the other with Hand of Glory. The one who cast Celestial Harmony, uses 1CP on the shooting phase so Skinks can reroll 1s to hit (Command abilty from GHB2020) while the other, who has casted Hand of Glory that has the same effect, uses 1CP if needed on NOT doing battleshock.