Issue with LCD volt meter

[jbrewton]

http://www.ebay.com/itm/3-1-2-Blue-LCD-Digital-Volt-Panel-Meter-DC-20V-/161045251073?pt=LH_DefaultDomain_0&hash=item257f0b8401

Ok, I know I can not run off common ground supply, but can I run it off a secondary rectified supply off the same transformer with 2 bridge rectifiers separate grounds?

Display will be attached to bottom supply and circuit display reading from top supply

[SeanB]

Simple answer is the 2 supplies are not isolated at all. They have a connection of somewhat complicated impedance and will pass current from any output or ground to any other output or ground of the other side. You need 2 electrically separate windings on the transformer with separate bridge rectifiers on each to have them isolated.

[mariush]

Or instead of buying a large transformer with two separate windings, you could buy a couple of cheap 7.5v ac adapters and then use the 7805 to regulate the output for your meters.
For example :
http://www.ebay.com/itm/US-DC-7-5V-1A-Switching-Power-Supply-adapter-100-240-AC-/181137467047?pt=US_Surveillance_Cables_Adapters_Connectors&hash=item2a2ca266a7

or http://www.amazon.com/Super-Power-Supply%C2%AE-5-5x2-5mm-Certified/dp/B00EA162QQ/ref=sr_1_2?s=electronics&ie=UTF8&qid=1393357527&sr=1-2&keywords=7.5v+adapter if you want to get it faster.

Hack them to remove the mains connector (remove the prongs and solder wires directly onto the pcb inside and you're in business. 
You may be tempted to get straight 5v ac adapters but don't, they're not regulated enough, the 7805 needs 1.5-2v above 5v to work properly so 7.5v dc from such ac adapter will be perfect.

[Excavatoree]

One interesting problem I found using the current measuring version of that meter is that if two panel meters are used to measure two currents that are not isolated, they must EACH be powered from an isolated supply.

That is,  I am measuring currents 1 and 2 from one common supply.  Each meter has to be powered separately, using two batteries in my case.  I would think that one battery could be used for both.