Actually I did a little diode research and you can get these
Schottky diodes which only drop the voltage by around 0.15V to 0.46V. They also put off a little heat so you need a heat sink. By comparison, standard silicon diodes have a forward voltage drop of around 0.6V.
Considering the y-connector you shared
@Frogslayer has a heat sink, I would say it it using either Schottky diodes or a MOSFET to protect polarity. It could also isolate the batteries from one another to avoid the fuller battery charging the flatter battery. It's hard to tell since the description doesn't really tell us anything. Nothing in the description about max voltage, max amperage or polarity protection.
Another simpler way would be to just put a fuse on the positive wire of each battery and then wire them together into
A diagram showing how to wire in parallel always helps (please disregard the voltage and amperage of the batteries in the diagram). Note that you don't have to wire it at the base of the battery terminals. You simply just bring the battery wires together and connect with plugs or solder - positive to positive and negative to negative on the batteries. Then connect the output positive to positive and negative to negative to the controller.
