Our district also required at least 2 years above level to accelerate one year, at least during elementary school. The testing wasn't a problem for DS who accelerated in 2nd (to GT 3rd which was 4th) and then again in 4th (to Pre-Algebra) even though he was not after-schooled in another curriculum. The school/district were fine with his ignorance of certain standard terminology and his idiosyncratic way of solving new problems. In a way, that helped convinced them because it was obvious that he hadn't spent a ton of time going through curriculum and prepping for the tests.
As others have already pointed out, I would not ignore the impact of writing and executive function demands. There also are usually a few students who are obviously ahead in a given grade, so testing 2+ years will ensure that your child will be in the range of the top ability students. The elementary curriculum spirals and is artificially divided into 6 years (K to 5th) so two years ahead is not actually that much. In our district, I believe that the two years rule is also not applicable once kids hit the Pre-Algebra level.
