Using two seperate balancers is not a good idea IMO because a balancer works by discharging high cell(s) in a pack. It works off the
relative voltage levels, not some absolute value. So, if two cells on one pack were at 3.7v-3.7v, and the other two in the other pack were at 3.5v-3.5v, each pair would be balanced, but not to
each other. The only way this works if there is a way for the balancers to talk to each other, like in the LBA balancers.
However, you
can use the 4s single unit balancer. From the picture you posted, I think it should be hooked like this:
