There are two methods you can use to safely parallel batteries.
To safely parallel batteries directly for more capacity, you must first ensure they are the same chemistry (lipo, li-ion, lifepo4, lead acid, etc) and the same voltage.
Never attempt to directly connect batteries in parallel of different chemistries.
It is also best if they have a 2 wire BMS (Charge and discharge through the same wires), since otherwise directly cross connecting them may defeat the overcharge protection.
By directly connecting the positive wires to each other and the negative wires of a 2 wire BMS, you can charge both batteries and discharge them through the same wires at the same time.
You must first however charge both batteries to the exact same voltage (fully charged is recommended as its easiest to reach) as directly connecting batteries of different charge states can cause huge currents to flow. Its best to verify with a DMM that they are the same voltage before connecting.
It is recommended to have an individual fuse or breaker for each battery, but not strictly required as the BMS should be able to disconnect the battery in event of over-current.
This means you can not directly connect a second battery in parallel as a backup unless its going to stay attached or only be attached when both batteries are fully charged. Use an A or B battery switch instead if you need to switch to a backup battery.
By using high power diodes, you can ensure your one battery won't discharge into the other and can even connect different chemistries of battery or even different voltages, although they won't share the load if you do mix voltage/chemistries and you will need separate chargers.
The diodes need to be rated for higher then your max battery voltage and the full current you expect to draw, ideally much higher. This will make them rather large and likely require heat sinks. The diodes only need to be on the positive wires, directly connect the negatives.
You also need to use additional diode or separate connectors for charging. If you use diodes they will need to be rated for the full current of your charger and batteries.