Two water signs read each other without words: intuition, emotional memory, an intimacy few bonds reach. You also catch each other's moods dangerously easily. Your work: don't drown together — someone has to remember to open the window.
One holds steady (fixed), the other flows (mutable): the fixed one gives the mutable roots, and the mutable teaches the fixed that changing isn't losing. Complementary — as long as neither caricatures the other as 'stubborn' or 'flaky'.