There are many cockpit functions in flightsim programs that use the CTR / ALT / SHIFT keys in combination with another key. In this way the number of functions accessible from keyboard can be increased.
Making the combination keystroke with one external switch is however not as simple as the circuits described above; I tried the above circuits with a dual pole relay (one pole for SHIFT and the other for a normal letter key. The result was not satisfactory. The keyboard needs to detect the SHIFT key being pressed 10 or more of milliseconds before a letter key is pressed, otherwise the SHIFT will be ignored. See illustration below.
To make this would require a one-shot IC like HEF4538 per
combination key function.