Originally Posted by nbTMM

This is the biggest load of crap I have ever read, and if that is representative of SMC's understanding of electrical engineering it is no wonder why people have so many problems with the batteries they sell. It beggars belief that a battery 'manufacturer' would have such a misinformed understanding of how their own products can be used. If this is not the case, I can only assume that they would publish misinformation like "the second pack sees double the resistance" to try to sell more higher voltage packs and discourage customers from connecting two existing lower voltage packs in series.

There are no issues with running two identical packs in series to create a higher voltage pack, and this is exactly what is happening inside a single physical pack with multiple series cells. The only difference is that by having two physical packs it puts the onus on the owner to:
1. Ensure that both packs are charged to the same level (read: fully charged) before use as a series pack. For two physical packs I would always recommend charging them individually with the same charger, or making a balance cable adapter so they can be balanced charged together in the same configuration that they are used in the car.
2. Ensure appropriate cutoff voltage is set such that no cell is over-discharged in use - the same has to be done for a single physical pack if there is a capacity mismatch between the cells. And yes you CAN use two packs which are not perfectly matched in capacity, just that the cutoff has to be set higher to prevent over-discharging the lower capacity pack. If they are the same brand and model pack, you should be golden. Just check your cell voltages every few minutes the first time you run it and when any cell is getting too low (<3.4V), your safe cutoff voltage becomes whatever the total voltage is at that moment.