How Important is it that 2 lithium battery packs in parallel have same voltage? More info in description?

I added a second battery to my xiaomi scooter but did not realize that the extra battery I ordered does not charge thru discharge wires! Xiaomi scooter has regenerative charging (charges itself a little when braking etc) so it charges the original battery when braking etc but the other battery does not receive that charge. So when I go a little ways the volts get more and more unequal. At least that is why I believe they become unequal anyway?  Is this real dangerous? I cannot find any info online about it. Just that they need to be equal volts when connecting them ..