joining up the USB outputs of a single phone charger to increase/amalgamate current capacity
You should NOT connect together your USB ports on your Powerbank, you will not get the results you want. 

The USB Powerbank you point to has 'Smart' outputs, that means each port has a controlling IC that sets the current limit for the port. This also allows the current limit to be set to a high value if the device asks (it does this by manipulating the D-/D+ lines on the USB cable). 

While you could certainly set up each port to the high current limit, there will be slight differences between each port that will result in current differences between the ports. If either of the ports does reach a current limit threshold then the other port will shutdown immediately (since you are over the current level from either port).

You should partition the current requirements of you project such that you don't overload a port. I use a very similar Powerbank in a small robot with R'Pi and have the servos on one port (I use servos as motors too) and the R'Pi on the other port.