1. A requirement of ippiThreshold_LTValGTVal is that the value of thresholdLT must be less than or equal to thresholdGT. I will note that, if this requirement is dropped, thresholding can be done in a range, in this case from thresholdGT..thresholdLT, To make this work, the threshold value OR condition in the IPP code must be changed (in that case) to an AND condition with swapped thresholdGT and thresholdLT parameters.
For float images there is currently no IPP function (I know of) to do thresholding in a range. For integer image, thresholding in a range can be done with ippiLUTPalette.
2. I rather question the usefulness of the IPP multi-channel thresholding calls. The have independent criteria for each channel and the result of that is ugly color changes. I think, useable multi-channel thresholding implies a single thresholding criterium, like on the RGBtoGray value. Of course, this can be emulated with a series of other IPP calls, but that leaves the current IPP multi-channel thresholding calls alone.