Just a short video on implementing a second input:I went back to using pushbuttons instead of motion sensors for easier testing for now. Now pressing the first button represents detecting a pedestrian, pressing the second detecting a cyclist. Only when both are pressed at the same time, the sounds are played.
I am currently experimenting with the millis() function: When the pedestrian sensor turns true (detects) I want it to stay true for a short duration. To achieve this without using delay(), the millis() command seems to be a good option. It is described quite well in this BlinkWithoutDelay example.