Intermodulation is a form of distortion caused by non-linearity in a circuit.
(But then all distortion is caused by non-linearity
).
Intermodulation
If you put two signals in to a "system", you hope to get only two signals out.
With Intermodulation distortion if you put in two signals of frequency f1 and f2, you will get out of the system not just f1 and f2 but also (2xf1 - f2), (2xf2 - f1), (3xf2 - 2xf1), (3xf1 - 2xf2), etc etc.
In a non-linear system you will also get harmonic distortion, where if you put in a signal of frequency f, you will get out signals at f, 2f 3f, 4f etc.
So having gone all around the houses with this, unless you are looking for perfection, don't worry too much about the intermodulation.
If you have a means of measuring harmonic distortion, I suggest that you use that I am sure that it will be good enough for most purposes.
At this point I must confess that I have no experience of 24 track tape systems, I am sure that someone here on ETO does have experience and will correct me if I am wrong in my assumptions.
(Afterthought, does a 24 track tape do some tricky multiplexing of audio signals on to multiple carriers?
If so the the intermod measurements may well be necessary.)
JimB