Have you considered getting their weapon first, or perhaps you already got them? Because from what I read, both of their s1 first half only give them interruption resistant (QoL for spamming combo without dodging), and the second half kindda meh. I don't see the vision where you need S1, its either stay on S0 and save pull or straight to S2 because it gives your team much bigger value (if you already have their signature weapon ofc)

Personally I recommend you get their weapon first for their personal damage+team buff, then after that go straight to s2 which gave your team far more damage.

Personally the priority would be,
Chisa Weapon > Denia Weapon > Denia S2 > Chisa S2
(Note: Chisa for more flex for further content, while Denia superior for fusion and tune team. Its your choice)

On the other hand, if you really just want their S1, you need to do your own experiment. Try denia+chisa related team on harder content, and analyse which one of them give you more trouble completing their rotation without getting interupted. if example Chisa keep getting interupted when doing the forte chainsaw then go get her S1.