NESE is a cool solution. Even more modular, which is always a good thing.
I imagine you’re asking about what the maximum voltage you can connect packs together in parallel is. It depends on how the pack is made (how resistive the connections are), the internal resistance of the cells themselves, and the maximum charge current (of the entire pack) that you are comfortable with.
For me I have 12S3P P42A packs. From online the internal resistance of the P42A cell is 0.02 ohms. The whole pack resistance is a minimum of (12 series) * (0.02 ohms per cell) / (3 parallel) = 0.08 ohms. When connecting two packs together there are two of these resistances in series so 0.16 ohms. From here it’s just ohms law. What is the maximum charge current you are comfortable with? For me I’m good with 10A from pack to pack, because it only is charging at 10A for a few seconds when initially connected together. This means I’m okay with a maximum voltage difference V = IR = (10A) * (0.16 ohms) = 1.6V from pack to pack. This neglects conductor resistance though so really the current would be less than 10A, but in general thats how I go about it. In most packs less than 1V difference would probably be fine.