Hey, simucube definitely leave this area grey, maybe for their marketing, the pedals are all the same, it's only one active pedal and 1 passive pedal. I see no reason why that wouldn't work, I struggle to find a difference besides simucube instructing buyers to purchase primary set for the 1st time buying or if purchasing only 1 pedal. The add on pedal comes with everything according to website, however I purchased the primary set as my 1st as instructed.
There is no difference in primary and add on. They are the exact same pedal. Difference is primary comes with power brick and simucube link hub. The add on pedal only comes with Ethernet switch. You can use the power brick and simucube link hub from primary to make add on a primary pedal. I purchased 2 primary pedals because the add ons were not in stock.