NanoVNA examination of stacked ferrite cores of different mixes studied an example stacked core scenario, presenting measurements of a stack of BN43-202 and BN73-202, 5t wound through both.
The article stated:
They are somewhat similar (but only somewhat) to two series chokes with the same number of turns, so you might expect overlap of the responses.
Above is the measurement of the stacked configuration.
This article compares the stack with measurement of two series chokes of the same number of turns.
Above is the measurement of the series configuration.
They appear quite similar up to perhaps 5MHz or so, lets compare R, X on the same graph.
The magenta and cyan traces are R and X for the stacked configuration.
Below self resonance peaks, they are very similar which hints that the stacked cores are approximately the sum of responses of each of the cores at lowish frequencies, but as resonance comes into play, they differ.
It is clear that the stacked cores do not behave as equivalent chokes in parallel, that proposition is very wooly thinking.
This is measurement of a specific scenario, and the results cannot simply be extended to other scenarios. I have not (yet) been able to create a simple model of the stacked cores above self resonance, I would suggest that for reliability, any proposed configurations be measured. Whilst in this case, the series configuration appears to have a better impedance characteristic, that might not apply to other geometries and mixes.
Build, measure, learn.
Read widely, question everything.