A high-low block is not legal at any time, in any order (high-then-low, low-then-high, simultaneous) in either FED or NCAA rules.
There is one distinction between the two: FED defines the "low" portion of a chop block as "at the knee or below", and NCAA defines the "low" portion as "at the thigh or below".
FED: 2-3-8 and 9-3-6.
NCAA: 2-3-3 and 9-1-10.
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