It is certainly preferable to run two batteries in parallel, rather than switching between them, but I think the necessity for diodes may depend on the design of the BMS.
The
OnBike instructions, and the experience of Cyclezee, suggest that the Ezee BMS has diode protection on the output, and their batteries don’t need additional protection. However the
instructions from Ping suggest that their BMS doesn’t, and recommends the use of diodes with their batteries - unless you’re very careful.
If you’re not sure about your BMS protection, on both batteries, and plan to run that configuration regularly, I would think fitting the diodes a good idea. A mistake could be very costly.