Here's what I mean:
<img src="http://i.imgur.com/KZl0g.png" border="0" />
The bar is between the pixels
To fix this, I recommend you make the game at 1x zoom, but use events to set the layers to 2x
You can do this with these events:
<img src="http://i.imgur.com/SWiIa.png" border="0" />
And you can zoom in the editor while you work on the game with shift + ctrl + mouse wheel