Users are sometimes a little confused what the InTNaT does.  Its job is to sum a crossover's outputs, do it differentially, and provide some protection for the NaTKiT and PC.

The InTNaT sums a crossover's outputs so they can be tested in one operation.  You might get some result like in this sketch.  You would expect the high pass output to be lower than the low pass because tweeters are usually more sensitive than bass drivers.  What quite will happen in the crossover region will depend on the particular crossover.

Some users have products that mostly sum like our sketch above but the odd model goes through the crossover transition smoothly.  When testing these oddities they purposely connect one output out of phase to get a response with a suck out like the others and so not worry the operator!  This is not an unreasonable thing to do as the NaTKiT tests comparatively.

You might have a circuit like this to do the summing but ...
if there were an open circuit in the common line of the low pass section then there would still be a path as shown by the red link; and the crossover wouldn't show as faulty, but ...
if you use differential amplifiers then there is no common path and the open circuit is detected.

When testing crossovers you are squirting the output of a power amplifier, perhaps even a mighty power amplifier, into the NaTKiT via maybe a single reactive component.  It has never happened yet but if something did go wrong the InTNaT would sacrifice itself and save the NaTKiT and PC.