The batteries could be used in parallel, but will fail. The problem is they are too inadequate in power output, If one string/ battery detects a cell going high under charge or regen it will simply cut out that then pushes the other remaining battery into over load and will shut that down. With both shut down the vehicle is then lacking control or drive. If a cell goes low under load , same thing. There is nothing wrong with having batteries in parallel, the problem is with having them too small and inadequate discharge capability in the first place.
We build Conversions in the UK built around 6 x 30Ah 60v nominal ex OEM cell blocks. Each of these stupidly high quality 30A cell blocks can readily deliver 150+amps,theoretically you could safely pull 900A so they are never stressed in normal use. And thats the ideal situation. Have enough Ah so the pack is never stressed. They are collectively monitored and generally we see cells holding within 0.02v in balance. But this isnt a cheap option. It is however a cheap option to our users who are all land owners or commercial farmers, and are fed up with repeated Discover cell replacements.
Do the conversion cheaply and inadequately reserves on the pack and it will have have big limitations. Its not a cheap alternative in the long run, but probably fine for occasional use. Thats not what we cater for .