Migrain/Arcane Conundrum

I was wondering if these two skills stacked on top of eachother.

P.s. i know migriane is spelt with an e

Yeah, it's spelled with and "e."

And they stack.

The cast time penalties stack up like 75% slower (I assume, looking at other % stackings) Because on the second one of the two, you take 50% of the remaining 50% cast speed, not just 50% off the total.
Otherwise you'd be casting at 0% speed, and be in a whole world of turd.

Btw, migraine's spelt with an 'e'