Summing Data depend on Index

Fred's contribution shows impressively that the use of Express strongly encourages creativity - chapeaux!

Here is a solution using programming (not working in Express)

Unfortunately user-written programs can't be written to accept a variable number of arguments. So I guess it would be better to rewrite the function to accept just one matrix and do the stacking outside of the function, maybe when calling it. That way its possible to deal with more than two matrices the very same way:

Prime 6 sheet attached