Our low voltage battery is the lower half of the pair as well.
Yes, a single balancer seems to be fine as long as you connect the centre note of all pairs together.
My batteries are only a couple of years old, so I will organise a wiring loom for the centre node as the first step while I’m home and fit it when I return in April. After that I will manually monitor the upper and lower banks and keep an eye on the balance for a while. If the banks gets out of balance I’ll look at fitting a balancer.