Will two AC devices in same socket share same low voltage in there DC circuits?
The DC output of USB charger will be isolated from the mains, sufficiently to protect the user from shock hazard (assuming a good quality unit, properly tested and approved). 

The treadmill controller is probably not isolated from the mains. 

It may or may not be safe to connect the output of the USB charger to one side of the mains. Certainly the wires, connectors and insulation are not designed to be safe. The other isolation barriers inside the USB charger may or may not be. I would advise extreme caution on this, and you should consult someone who is familiar with mains circuit safety and can look over your proposed setup. 

Keep in mind that if you tie one side of the 5V USB to the mains and then connect a micro into the 5V USB power and plug your computer into the USB port of, say, an Arduino you run good chance of blowing up the computer, Arduino etc. and/or giving someone a nasty (possibly lethal) shock. Everything connected to the mains is then not 5V with respect to earth, but mains voltage wrt earth.