Try this:
a = rand(250, 2); % Sample data
% Make kernel to do the averaging.
kernel = [1;1]/2;
% Make every row of b the average or two adjacent rows of a.
b = conv2(a, kernel, 'valid')
It give you exactly what you're asking for. With your example data:
a =
4 3
2 2
3 3
4 2
b =
3 2.5
2.5 2.5
3.5 2.5
