so far I've come to this:

E2 goes along with E5 - the Giver and the Accumulator. Opposing/complementary.
E4 goes along with E8 - The Sensitive Soul and the Brute.
E7 goes with E1 and vice-versa - not sure of this one.
E9 goes with E6 and vice-versa - also not sure.
E3 goes with E2 and vice-versa - maybe.

Only the first two pairs of complement relations I've been able to see in real life so more research must be conducted before I have a final table of Enneagram Duality but the first two pairs are correct.