You could use cell arrays. Cell arrays allow you to store data of different types (strings, ints, doubles, arrays, etc.) all in the same data structure. They also allow different sized arrays to be stored together.
%Get in correct directory with Excel File
[status,sheets]=xlsfinfo('blahblah.xls');
a=numel(sheets);
%be sure to initiate the cell array, just like you would for an array
P = cell(1,a);
for d=1:a
[b,c]=xlsread('blahblah.xls',d); %Read excel file
x=b(:,1); %Choose columns for outputs
y=b(:,2);
%store the result in P
P{d}=x.*y;
end
