I suspect that what Bill was saying was along the lines of 'Don't expect to get more than half an axial fan's free air flowrate spec when using it with a 2" heatercore.' (I don't really know though since I don't know what article you are referring to.)
I agree that stacking heatercores is not a good way to go, mainly because the fan noise required to get good performance is too high. Even if the air flowrate only drops to 85% (of single HC flowrate) with two heatercores stacked, (and reasonably quiet fans) the air will be heated so much by the first HC that the second HC won't be able to dump much heat.
It's certainly possible to get improved performance with stacked HC's, just not worth the noise to me.
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