The "When I press" triggers are independent of each other, so you'll need to trigger off of one of them, and then check for the other within the command.
E.G.
When I press keys: Left Alt
Start Loop While : Keyboard Key 'Alt' Is Pressed
Begin Device State Check : Left Mouse Button Is Pressed
Write [Blue] 'Alt-click detected' to log
End Condition - Exit when condition met
End Loop
If you also want to Alt key to perform a function by itself, you can add those actions underneath the loop, so that they will be executed when the Alt key is released.