It’s not a silly question. In general, in DSP, we tend to avoid like the plague the use of any IIR filter with an order higher than 2, for preventing some potential stability and accuracy issues. So, most of higher order IIR filter designs are implemented using parallel or serie first/second order filter sections. In my knowledge, that’s how several commercial EQ plug-ins implement their high slope filter designs.
Of course, having two filters operating in serie will display a change in the phase response, but I’m not sure you will get any specific issue by doing so. The only problem that you will have to solve is how to set the Q factor in your serie filter designs. What is the definition of a 24 dB lowpass filter with a Q = a specific value ? Do we need to use that same Q for the two serie filters ? Do we need to use two different ones ? I’m not sure there is a consensus on that. For example, I think I have seen somewhere that the curve isn’t the same on FabFilter Pro-Q and Pro-Q² for a given lowpass filter with a slope of 24 dB and the same frequency+Q. However, the maximum magnitude of the resonance has to be the same everywhere and whatever the slope.
Thanks for your nice comments