Seija + Shion
Seija turns Shion's unluckiness into good luck
Alternatively Seija + Hina = Good fortune for the same reason as above

Actually now I think about it, Seija can be paired with a lot of 2hus for interesting combinations:
Seija + Rumia (manipulating darkness) = manipulating light
Seija + Flandre (ability to destroy anything) = ability to fix anything
Seija + Yamame (ability to infect with any disease) = ability to cure any disease
And so on