Different plants, and indeed the often the same plant, harvest nutrients by different means. Stems tend to get more nutrients directly from the water column, when crypts, swords, and other rosette type plants tend to get much of their nutrients directly from the substrate. Because if this, fertilizer both directly in the water, as is the case with normal Flourish, and fertilizer in the substrate with root tabs feed different plants, or alternately allow a single plant together nutrients from both places. You can and should definitely use both.
Also, did you mean normal flourish? Flourish excel has no ingredients in common with either normal Flourish or root tabs.