Heart of the Sea + Water Supremacy (Hold Breath (EX))

If a human character has a CON of 12, the alternate racial trait Heart of the Sea, and they are a Water Elementalist wizard with Water Supremacy, how does that work for calculating the number of rounds they can hold their breath?

Heart of the Sea says "they can hold their breath twice as long as normal" - but Water Supremacy says "you can hold your breath for a number of rounds equal to four times your Constitution score" - does the x4 from Water Supremacy replace the Heart of the Sea, or do they stack somehow?

Water supremacy replaces the normal x2 with x4, heart of the sea doubles that to x8.

Thanks! That's what I needed to know.