Though I doubt that it is software, you could try to install and run KeyEvent. Showing the make and break codes for each key.
If you press one key, and keep it down, and then another you will see that it you wil get independent codes, and two different up's
If you do the same with the two shift keys, you will see that they act as parallel. Press and hold 1, the press and hold the other, and relase the first then the second, and it acts exactly as if it was one key. The yellow arrows does not (and if they work, the ctrls act in the same way as the shifts(