Your ESC should have two male deans connectors like the ones in the first photo below. The second photo shows a male Deans connector on the left, and a female Deans connector on the right. You need a female Deans connector with the solder tabs shorted together. You plug the shorted female connector onto one of the male ESC connectors, and you plug your battery pack onto the other ESC connector.

Alternatively, you could just short out one of the male Deans conectors on the ESC, but then you wouldn't be able to go back to running two packs in series again without undoing the modification.