Please see the documentation section on Matrix Indexing (link). You need a second index to address your matrices. What you do depends on the result you want.
Both of these work. One should produce the desired result (assuming ‘M’ is your (64x16) matrix):
DataA = M(:,1:2:end); % Result Is (64x8)
DataB = M(:,2:2:end); % Result Is (64x8)
DataA = M(1:2:end,:); % Result Is (32x16)
DataB = M(2:2:end,:); % Result Is (32x16)