If your need is to place the single digits to create a bigger number if you would like to avoid passing through string conversion (as suggested by Andreas) you can use sums and multiplications to create the number.
M(1,1) = [a b] * [10 1]';
% For more elements
a = 1;
b = 2;
c = 3;
d = 4;
M(1,1) = [a b c d] * (10.^(3:-1:0))';
This means a * 10 + b * 1 = 12. In the second example I'm extending the notation to allow the definition of more terms without writing a * 1000 + b * 100 * c * 10 * d * 1.
If your need is to store two values (a matrix) in another matrix you need to useCell Arrays: