The problem with your setup is that you're trying to cheat the hardware; USB, and more specifically the Kernel support for USB on the tablets, are designed with power negotiation protocols and handshakes that have to happen in order for a device to be "powered up". Essentially the way it works is:
1) When in host mode, the tablet keeps a very small amount of power available on the USB port.
2) You plug in a USB device. The device uses that small amount of power to send a message to the tablet saying "I need X amount of power to work properly".
3) The tablet (kernel) receives this information and decides based on the hardware information whether that much power is available to put out the USB port.
4) If the kernel decides it's okay, then it turns up the USB port, subtracts the amount of power it's just allocated to the device from the total power available, and goes back to 1)
5) On the other hand, if the kernel says "nope, not enough power available" then it refuses to activate the device (to prevent USB over-draw and power problems) and goes back to 1).

It's a bit more complicated than that in reality, but that's the concept. The problem with your homemade Y cables are that they don't take this power negotiation into mind; that is, although technically you've provided enough power straight to the device itself, when the tablet tries to initialize it the device still asks for it's power level and the tablet still thinks (rightfully so) that it doesn't have enough power to activate the USB device on it's own.
Anyone who's had success with this type of Y cable is using it with either 1) a device that doesn't really need an external power source as teh tablet would be capable of powering it on it's own, but the power source is there for other reasons (longer battery life,e tc), 2) a device that does not report it's real power consumption level during negotiations 3) has a custom hacked kernel with USB power management disabled.

The proper way to do this is to use an externally powered (or battery powered) USB hub. The hub has logic circuits in it that will allow for proper power negotiations with the tablet (essentially the hub connects and tells the tablet "I've got this much power available to me, so use that as the value to check against for anything plugged into one of my ports").

Thanks,
-Rob