First of all, you do not have a cell that contains an array.
You have a cell array with 3 cells, each of which has a single row vector in it.
Anyway, here is code to do what you asked.
% Define the cell array A, which has 3 cells.
A={[1 2 3] [4 5 6 7 8] [9 10]}
% Get two random indexes in the second cell
randomIndexes = randperm(length(A{2}), 2)
% Get two elements from the contents of the vector in the second cell using randperm().
twoElements = A{2}(randomIndexes)
% Now concatenate them into a single row vector of class double.
B = [A{1}, twoElements]