There is no law on the books that would prohibit a NRST on top of an income tax now. However, once the FairTax is the law of the land, taxation of income, estates and gifts will be prohibited.
No congress in their right mind, would totally abolish the income tax system without a successful, proven, revenue generator in place. Once the success of the FairTax is realized...taxing "income" will be irrelevant and repeal of the 16th Amendment would fly through the states for ratification. Repealing the 16th will be easier, not harder since the amendment no longer has any real meaning - it is an anachronism.
Of course, the reality in the latter case is that the evildoers of the world will put a sword at your throat to make you plow for them, and the reality in the former case is that the taxpayers will end up paying both income and NST (as in Canada).