I don't think it's a great idea to connect the 5V of the tablet to the external 5V.
my DIY Y cable goes this way:

(X : not connected, --- : connected)

Male USB A (USB power pack) to Female USB A (device):
5V ------------- 5V
Green -X X X- Green
White --X X X- White
Gnd ------------ Gnd


Male Mini/Micro USB (tablet) to the same Female USB A (device):
5V -X X X X X- 5V
Green --------- Green
White ---------- White
Gnd ------------ Gnd

so on the Female cable you should only have Gnd connected to both cables. Data connected to the NIT, and 5V connected to the power pack.

The downside is that you can use this cable only with a power pack, it can't draw power from the tablet.
If you know what you're doing, you can put a low voltage-drop diode (Schottky?) on the 5V/5V cable connected to the tablet, but you still lose some hundreds of milliwatts. I don't know what is the exact output voltage of the USB host port on the tablets, but if it's too low it can make your USB devices not work.


I did this mod, and mine works.

[Edit] is your NIC powered by something else than USB though? if you don't connect the power pack's 5V to the NIC, it may not be supposed to work.