This program demonstrates a technique for detecting when two keys are pressed "simultaneously." Of course the detection is not perfect, but it's a reasonable approach for many Scratch projects. The idea is to set flags when keys are pressed (set their values to 1) and remove those flags (set their values back to 0) after 0.1 seconds. The key presses themselves trigger a "wait for" block that controls a stack of actions. The "wait for" condition is that both flags are set to 1. If this is true, the blocks in the stack are executed and then the flags are reset to 0. This approach could be extended to any number of keys and actions.
Jesse Heines, UMass Lowell Performamatics Project, Sound Thinking Course April 19, 2015
