Example (Click to enlarge):
A: This is Z-fighting, they are basically fighting for their space in the z axis (from camera) and this is causing them to have the look of being on the exact same coordinates.
Fixes:
Easy way - Move them further apart, if possible.
Other easy way, but sometimes could be a problem - Adjust your camera's clipping planes. This can be done by going to View>Camera attribute editor, in the viewport (may vary by Maya version) From there the clipping options are under the Camera Attributes section. Try setting the near clip a tad higher, and the far clip a tad lower, you'll usually only need a very small change to fix it.
Example (Click to enlarge):
