Wired in series. The concept remains the same for whatever size battery. I personally would only use two that match. Check and recheck polarity. Make sure all your connections are well insulated. Make sure there’s no chance of touching battery connectors as you plug the two 1s batteries in.
Very important that they match in capacity (including capacity loss during wear), otherwise while your osd says you “2s” battery has 7v left it could be 4v in one pack and 3v in the other, etc.
Careful testing of potential pack matches will turn those issues up safely though.
That’s a very nice compact adaptor, mine is rather larger.