Curious "open ground" when using testers in series
The way those cheap 3-lamp testers work, they can't possibly be interacting with one another.  If they are acting that way, then either they are pieces of junk (Made in China?) or they have exposed a wiring problem. 

There is also no benefit whatsoever to using two such testers at the same time.  Test one outlet... Then test another.  

Keep in mind 3-light testers are just "idiot lights" only slightly more useful than the Magic 8-Ball, and fail to detect a wide variety of faults.   You've seen neon testers with two test probes; think of a 3-light tester as three of those.  


hot-to-neutral (yellow, the one that is on for "open ground")
hot-to-ground (yellow, the one that is on for "open neutral")
neutral-to-ground (red)