Eternal Chilling Darkness does not stack with Fey Energy Tap IIRC, which suggests to me that it likely would not stack.
I would guess that they would be on separate timers though, such that you could trigger one to re-up the temporary spell points with one while on timer to trigger the other.
I'm not sure what build would want both - that would require not wanting the exceptional DC from a two-hander and steady use of both cold/negative and fire/light spells. That's a big stretch for a PM DC caster. Maybe a Druid might consider trading a DC for this?