Well, if both keys are down, the conditions of the first two event will be met, so both of them will run setting "Left" and "Right" to who knows what.
You could try this instead, but I don't know if it's really necessary since the "Key is down" conditions are pretty much "Left"/"Right" booleans themselves, you should be able to do it without using extra booleans:
+Right arrow is down
+Left arrow is not down
[ul]
[li]> Set Right to true[/li]
[li]> Set Left to false[/li]
[/ul]
+Left arrow is down
+Right arrow is not down
[ul]
[li]> Set Left to true[/li]
[li]> Set Right to false