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.
Not using a Y-cable. No.
If your laptop is 5.1V and the power bank is 5V, what happens then? What happens if it's the other way round? Or 5.0V versus 4.9V? Are you going to be tweaking all your powerbanks and laptops to be millivolt compatible to each other?
Not to mention that opening up a USB cable will already demote it from USB2.0 to 1.1, as the shielding and exact twist vs power pair twist and distance and what-not are very important for the 480Mbit/s (burst) speed signal integrity. If the signal gives too many errors your hub and/or PC will auto switch back to high-speed 12Mbit/s. If you are fine with that, then that's not stopping you, but you're talking about hard-disks, so I'm guessing you're not fine with it. Just to be clear: That would be 12Mbit/s for everything on the hub combined. In the case of 12 ports with a HD each, which are all in use that's 1Mbit/s per HD = not even enough for HD video stream.
Then we can also throw in the limitation that if a USB hub manufacturer knows that a USB port is only allowed to supply 500mA they are NOT going to be designing it such that it can handle draining many amperes. If they put protections in they will be at about 750mA at best. If they didn't anything over 1A or so can cause serious unexpected problems. If I'm told "This connector is protected 100% of the cases to never give more than 500mA", I'm not going to waste my own time and money designing a PCB that can handle 5A, because that costs significantly more real-estate. Though I will definitely add my own protection, but that's more a condemnation of the reliability of the info managers usually give.
For this purpose there are hubs you can buy that include a power-path manager and a 2.5mm (or other size) DC jack 5V in, which allows you to source an alternative 5V at 2A or 4A or whatever to the hub separately and safely.
This will then stop the hub draining from the USB input and sourcing the total current to all USB ports equally. Which will still be limited to their specification internally, as functional USB ports are not allowed to just pump out 2A, that's in the official USB rules, so a hub that is commercially sold will stick to those rules and output at most 500mA normally, or if it's expensive enough to contain powered-negotiation up to 1A.
EDIT: As a side note:
Be aware that most dual-power hubs do hard-connect the ground of the incoming voltage to the ground of the USB. Fine, you may think, but please do check that the USB ground then is compatible with the ground of the voltages, if you use the same power bank for everything. If your powerbank happens to make a +5V for the USB and then a -5V to get 10V that gets smoothed into 9V, your tablet will "live" at -5V ground compared to USB ground = bad. Same goes for whatever a tablet manufacturer put in there to generate the USB 5V output.
It's very, very unlikely anything will happen, but unlikely is not impossible, whereas a multimeter first on resistance measurement, then on voltage measurement to compare would give you exclusive proof.