USB power in Parallel, increasing the maximum current using a Y splitter idea[POWER ONLY]
The problem with this approach is, as with every other "connecting two power supplies", that the voltages are not the same. The nominal voltage is 5V - yes, but the actual voltage is different. One might be 5.05V and the other 4.8V. Both are in the spec for USB 5V. 

But if you connect those two together, one will start to drive the other because it has a higher voltage.

Why the normal Y-Cables work? You plug them into the same device which has a single 5V source, so the voltage is really the same. Only each port is current limited.

And I see another possible question coming:

But there are USB hubs which use an external power supply, which is also a different voltage than the original port - how does that work?

Right, but those use either completely the external power supply or have some switching logic inside to switch from bus-powered to self-powered mode. For example the TPS2070 from TI.

So I would not connect two seperate power supplies to a single cable as long as I don't know the exact inner workings and can be sure that nothing will go wrong. It might work (if the things are reverse current protected for example) or it might not.

But I'm not an expert on USB power, so maybe I'm completely wrong.