It should be treated just like the 1 and 3 volume NRRD files in slicer: as a scalar image (in this case with two volumes), and the same as the 2 volume NIfTI. At the moment, dragging and dropping a multi-volume NIfTI or NRRD just displays the first volume of the series, but it seems odd that a 2-volume NRRD is treated differently than the 1 and 3-volume files. Ideally, it would be great if all the volumes are loaded and the user could choose which volume is displayed, just like ImageJ, FSLeyes and MRIcroGL. Rapidly cycling through raw DWI and fMRI volumes is useful for detecting head movements, reconstructor errors, etc.
Here is a link to minimal dataset of 1/2/3 volume image in NRRD and NHDR.