When you readtable you should not assume that all of the entries are numeric, or that all of the variables are present in both tables and in the same order, or that the tables have the same name of rows.
After you have validated and normalized the two tables, table2array and add and array2table the results. When you convert back to table form you will need to decide on appropriate variable names.
If you are sure that everything is numeric and same order.... then perhaps you should be using readmatrix instead of readtable.
