You are correct. Filters like this stack linearly, which means the Fdeck does filter 24dB below 35Hz. If I'm filtering 12dB per octave below 35Hz then adding another filter that's 12dB per octave below 70Hz, what happens to my first filter? 35Hz is below 70Hz, which means you will get another 12dB per octave filtered off there.Now you are just trying to confuse me (easily done) , FDecks filters are 12 + 12 filters which mean 12dB at 35Hz and a variable filter at 35Hz to 140Hz with a 12dB slope as well, this makes FDecks filters 24dB below 35Hz.
Two ways to write the same thing, in my opinion.