You hook them up in parallel. They have to all have the same cell count and charge percentage. No using a dead battery with a charged one. They make connectors for them or you can make your own. Here one for example on Amazon.
Hobbysky 2pcs No Wires 1 XT-60 Male to 2 XT-60 Female XT60 Parallel Battery Connector
https://www.amazon.com/dp/B01HZ5VPO2/ref=cm_sw_r_cp_apa_vUxVAbSQ1WK9Y
Basically turns your two 1300 3s battery's into one 2600 3s.
The other option is to connect them in series. Would turn your two 1300 3s batteries into 1 1300 6s battery. But what you were asking for would be parallel connectors. I used the one I linked above on my blunt nose versa. Put an extra battery up front for nose weight so I got one of these and double my capacity. 30 minute flights compared to 10-15.