I have tested on the water the 2 batteries indide the POD, and it’s important to know that the device does NOT float whit both bateries inserted.
This is in my opinion a really important factor to take in to account.
Confirmed with the manufacturer:
It floats with only one? 
Of course it does! (It has to) actually, it’s a key feature in case you miss it on the water. The orange cap (Decathlon version) is just for this and make it visible in the water, this floating spec is detailed on the product detail page.
Thanks to this I could recover it one time, after not fixing it properly.
Probando el motor eléctrico para Paddle y Kayak de Decathlon (youtube.com)
Motor eléctrico Kayak / Paddle surf | Decathlon
For the cable to connect 2 batteries, put on AliExpress XT30 parallel cable (2 female and 1 male) and you’ll get ton of options.
Hello, ludwig
your project looks amazing. I’ve decided to build a single 6s spare battery to extend my kayaking trips.
Does the original battery use a smart BMS?
Can I use a standard 6s 20 amp balanced BMS? Thank you."
The original BMS is also just a “dumb” BMS with balancing and shutoff when over- or under-charge / temp.
