There are a couple of options.
Option 1
The simplest is to consider each pair of batteries as one 12 V battery, and to just balance four pairs. One four-battery balancer only.
You may want to check how the individual batteries in each pair are doing every so often, but this at least balances the pairs and that's a pretty decent start.
Keep in mind that each 6 V battery is made up of three 2 V cells and we are not balancing down to individual cell level, so in effect this is what each of my battery banks has - one balancer per group of six cells, it's just all six of my cells are in the same 12 V battery rather than in two 6 V batteries.
Option 2
If you were really thinking you wanted to balance all of the 6 V batteries with each other then you can cross connect these balancers but for eight batteries in series it would require three balancers.
e.g. you can connect them as follows
Balancer 1: Batteries 1,2,3,4
Balancer 2: Batteries 5,6,7,8
Balancer 3: Batteries 2,4,6,8 (or batteries 1,3,5,7)
It's obviously triple the cost to buy three balancers and prices seem have gone up a fair bit since I got mine. So it will come down to what these are being used for and how critical you consider this balancing to be for your application.
I think what I would do is option 1, then monitor the pairs for a while and see how they go.
The caveat with all of this is if you have a bad cell somewhere, or a bad battery, then a balancer is not going to make up for that.
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