For synergy you want one that debuffs and one that takes advantage of the debuffs. And that means one that isn't a witch IMO. You could maybe stretch a point and make that second one a hexcrafter magus or shaman or spirit guide oracle.
Are you more concerned with them being identical or having good synergy. Or would them being very different from each other be a plus?
I recently played two characters who were twins, I made them a Bloodrager and a Sorcerer with the same bloodlines and gave them the same alternate racial traits, background traits and stats, except the floating +2 stat from race (Half-Orc) went into STR for the bloodrager and CHA for the Sorcerer.
EDIT: I know that's not super helpful for Witches, but hopefully it can help thematically if not mechanically. My characters covered different combat roles, which - as avr said - will probably be more effective. You might be able to get the theme of twins while making them different classes by going with something like a Hexcrafter Magus and a Witch.
This falls under other suggestions: Half orc blodrager and skald (totem, bull) bring the pain. Feats amplified rage and power attack. At third level Bloodrager +12 str and skald +10 when raging.