I can imagine an Odin+Eirika duo...

If Odin gets Def/Res Link B slot and uses a positioning assist on Eirika, Odin gets +30 damage from his weapon and Eirika gets +12 total in all stats... A deadly combination indeed. Eirika can also cover the weaknesses Odin has. Builds I thought of:

Odin (+Atk -HP)

Odin's Grimoire (+Eff)
Swap/Reposition
Glimmer

Close Counter
Def/Res Link
Atk Smoke/Spd Smoke
Quick Riposte Seal

Eirika (+Atk/Spd -HP)

Sieglinde (+Eff)
Any positioning assist
Any special of choice

Distant Counter
Wrath/Chill Spd
Flexible C slot
Flexible S slot