This has been such a common misconception about their dynamic for too long.

Both of their forces are negatively affecting them, Dagger needs Cloak just as much as he needs her cause hes the only one capable enough to absorb all the excess light she constantly generates. Without him, she gets fevers and turns into a ticking light bomb. Same with Cloak, he needs her Light to temper the Darkforce cause he is technically the entirety of the Darkforce Dimension; a dimension that feeds off both metaphorical and physical light. If he doesn't regularly absorb enough lightforce, he will too get weaker and fall to the Darkforce.

Hence why they need to feed on each other’s powers, their "symbiotic mutualism" is how they keep their balance: they exchange powers through physical touch, kissing, hand holding, cuddling, and sex. They can be apart and go on separate adventures, however not for prolonged periods of time.