Ready Made
As a first step, look for something that somebody already offers that will do the job (you can't be the only person who needs to charge the tablet while something is plugged into it). Third parties sell solutions when there is a need. If you can't find a commercial product, then step 2:
Build Your Own
Approach: What you want to do is the reverse of what is commonly done for external hard disks, but you would do it the same way using a "Y" cable. As background, these Y cables became a popular solution for USB 2.0 external hard disks because the power available from a USB 2.0 port often isn't adequate. Two of the wires in a USB port are for power, the remaining ones are for the signal. The Y cable connects a second USB cable to the first that just picks up power from a second USB port. The signal is transferred at one port but two ports are providing power.
You want to transfer the scanner's signal at your one port but add power to that port at the same time. The solution is to use the same kind of cable used for hard disks, but use it backwards; the tablet will be at the receiving end instead of a hard disk.
Note that the "Y" cables are much more common for USB 2.0 connections, so you need to check the specs for all of the components. You can use this approach for devices other than a scanner. If the tablet has a USB 3.0 port and you are connecting a high bandwidth device, the Y cable and any adapter cables on the signal path need to be USB 3.0 in order to contain the connections needed for high speed data transfer. USB 2.0 components will work but will limit the data transfer to USB 2.0 speeds.
Designing the solution: The goal is to create a Y cable with the right connectors to match your devices. How you accomplish that depends on what adapters are readily available to you. In your case, the scanner cord has a male micro USB connector as does the power pack. The tablet has a female micro USB connector. It becomes a puzzle to create a "Y" cable with a male micro USB connector to go into the tablet and two female micro USB connectors to receive the scanner and power cords.
I did a quick search at Amazon.com and found this as an example of a "Y" cable:
It has a male micro USB connector to go into your tablet but two male USB connectors where your scanner and power pack need to connect. So we need something to turn the male USB connectors into female micro USB connectors. A little searching finds this adapter:
The instructions for the Y cable will identify which connector is the extra power connector and that is the side of the "Y" you plug your power pack into.
I was surprised that this could be put together from only three parts. Often, you need a motley collection of connector format changers, gender changers, etc.
