hallway lights not working
First thing, you should probably shut your breaker off if you haven't already because it sounds as if you have some wires crossed here. 

From your description it sounds like you have: 


Lights
Downstairs Dimmer Switch
Upstairs Dimmer Switch


This is requires a 3-way switch circuit... but I'm not able to determine which one you have. 

Generally, Black and Red wires are the "hot" wires (or your line wires). White is your neutral, and of course green or copper is ground. If you're getting voltage flickering on the white wire, you're likely registering return power on the circuit. If it's flickering without you touching the switch, you could have a lose connection somewhere. 

You shouldn't have only 2 black wires in one of the boxes. There should also be some white wires connected via a wire nut, as well as one or two additional red wires. Without knowing more, it's difficult to determine what type of 3-way switch wiring you have. You may want to check out the diagrams here to determine what you have:  http://www.do-it-yourself-help.com/3_way_switch_wiring.html 

Side note: Having 2 dimmers in series on the same circuit isn't a good idea. The 1st dimmer can only increase the power to the total level of the 2nd dimmer. Which is why normally when you have two switches on a dimmable light, only one of the switches is a dimmer. 

Good luck!