Even though what i will say is complete speculation and may be made totally irellevant when the game will be released, i like the idea of a Tag Team Fighting game. However, considering the title " Antinomy " i have some of potential ides for the duos, where they are similiar, but also the opposite of each other.
Reimu and Mokou : Well leaving aside ULiL After Story as a connection, both of them got no real purpose in life (Reimu being too carefree, Mokou being immortal), both of them are humans regarded as monsters due to their level of power.
Marisa and Byakuren : Marisa being a brazen human seeking power and immortality through magic, while the other one is a humble magician who already achieved youth and power through magic but, through buddism, try to stay " human " in a way. Also, Byakuren is frendly with youkai while Marisa think that she can't trust them.
Kasen and Miko : The first pretend to be a hermit to meddle with and befriend humans, while the latter became an hermit to overcome her humanity, while both having " darker " sides to their past.
Koishi and Kokoro : Both seek their emotions, Koishi is doing it in a really carefree way (because she can't think of another way anyway) while Kokoro is doing it seriously, in a more "scholar way". Plus, both are archnemesis ( i believe Koishi still has the mask of hope ).
Nitori and Shinmyoumaru : Shinmyoumaru is stated to be quite childish and na?ve, so Nitori could trick her into using her mallet for her in exchange of an invention that makes her grow taller ( as she is obsessed with that ). Nitori is also quite Haughty and think herself superior to others, so she seems suited to use the inchling as a mean to gain more power.
Futo and Ichirin/Unzan : They may share , like Alice and Marisa a " bad " relationship, as rivals from the Taoism vs Buddhism feud. They are both underlings of the represent on both religions, as well.
That leaves Reisen alone, as there is 15 characters (Unzan excluded). Whether Reisen will be in AoCF is debatable as she was a PS4 exclusive, so unlike she is paired with another character added to the game (though her general personality don't make me think of a possible duo), i don't have any ideas regarding her.