A board-to-board connector assembly includes a connector interposer between first and second connectors each having a connector housing holding contacts and having a card slot for interfacing with a circuit card edge of a corresponding add-in card. The connector housings have board guides configured to engage the add-in cards and guide mating therewith. The connectors have latches configured to latchably engage the add-in cards to secure the connector to the add-in cards. The connector interposer electrically interconnects the first contacts and the second contacts.