A board-to-board connector assembly includes first and second connector assemblies and a connector interconnect between the first and second connector assemblies. Each connector assembly includes an outer shell and a connector received in a cavity of the outer shell. The connector has a connector body holding contacts in a card slot, which receives a circuit card edge of an add-in card. The outer shell has an outer board guide configured to engage the add-in card and guide mating of the outer shell with the add-in card. The outer shell has a latch configured to latchably engage the add-in card to secure the connector to the add-in card. The connector interposer electrically interconnects the contacts of the connector assemblies to electrically connect the add-in cards.