How to use two Demux to index a matrix layout of registers
You can not do what you want with just 2 MUXes. For your example, you might use one 4:1, 8-bit, MUX for each row to select the register from one column in this row. A fifth 4:1 MUX selects the output from one of the rows.

That's a lot of multiplexers, which is why we typically use a multiplexed data bus for a register array. Every register has 3-state outputs and the selection logic allows only one register's output drivers to be active at any point in time. Then you can just connect the data outputs of all of the registers together