Powering a Canon DSLR via USB
Yes, step up the voltage from the power bank and it has the power ability to power the camera.

However you need to build the DC-DC converter so that the ripple on the output is small enough that the camera can cope with it and make sure the DC-DC is efficient enough in the energy transfer.

My experience of set up converters is that the output ripple is always hard to bring down, but using LTSpice or similar simulation software you can tune it easily enough. Simulation will also make sure you keep efficiency high enough.

Another concern will be how the power bank supplies the power, but that's just making sure you draw from both USB ports after doing whatever handshaking is required (some USB things only give power out of the USB socket after some handshaking procedure). The power bank may also turn off it's output power after a time, or when the voltage crosses thresholds (a common feature in portable USB power bricks to stop charging automatically once a device is fully charged).

If you are fixed with using that power bank as the supply, then some boost DC-DC converter would be required, so I would say that is the best way within the limits you've put in.

(Oh, and be aware that using this Frankenstein power pack may invalidate any warranties on the camera and the power pack.)