As written, as long as you know how long either one of the inputs was, yes.
P = @(x, y) [x, y];
x1 = 1:5;
y1 = 6:10;
z = P(x1, y1)
x2 = z(1:5); % I know how long x was, extract that many elements from z
isequal(x2, x1)
x3 = z;
x3(end-4:end) = []; % I know how long y was, remove that many elements from z
isequal(x3, x1)
