First off, where is "here" that there are no encoders available.
Second, there is no way that I know of to reliably get two players 8 buttons each from a keyboard hack.
The best I would reasonably hope for was two players six buttons each from a 16x8 matrix, (or maybe 7 buttons each from a 18x8 matrix.
Third, you don't have an 18x8 matrix, rows 9, 10 and 19 are empty, so you have a 15x8 matrix.
That leaves you with only enough for Player 1 and 6 buttons and Player 2 and 5 buttons and admin buttons.
Fourth, I don't know if the above statement is correct, as I only count 59 instead of the 104-keys. The arrow keys are missing, as are the function keys, and a lot of others. If any of these are on the empty rows, that greatly increases what you can do.
Fifth, even though you have mapped out the matrix, you still may have firmware that prevents more than 8 keys from being depressed at the same time. I can recommend keys and you can test it.
Let me know if you want me to recommend keys based on what you have (2-Player, 6 and 5 buttons) or if you want to test and post a new matrix, or if you want to figure it out for yourself at
http://www.mameworld.net/emuadvice/keyhack2.html