Electronic – Combining two exact USB battery packs in parallel to increase current in a simple fashion

I disagree with Chris in that you don't just want to shorten the cable. It's not the length of cable I'm worried about, but that tiny connector you don't want to replicate. The connector and the wires immediately coming out of it look to be intact. Replacing that will be a major pain in the butt, so I'd go to reasonable length to preserve that piece.

Since that piece is now short, you'll have to make a splice. Just realize up front you're not going to make a splice in such small cable that is anything like the size of the cable. The splice will be big, fat, and ugly in comparison. It looks like it can be made to serve as the strain relief too, so that helps a little.

To splice such tiny wires, see if you can sortof stick the stranded ends into each other. That may take some persuasion under a magnifying light, and then some weighty objects on your bench to hold them in place while you solder. This won't be easy, but it should be doable. If that just isn't practical, give up and bend each wire into a U and hook the two U ends together, flatten them together with a needlenose, then solder.

Once you have the connections made, wrap each one individually in a small piece of electrical tape, then wrap the whole bundle in electrical tape so that is looks like one fat section of cable. Like I said, big, fat, and ugly, but it should work. All that tape will act like a strain relief, so that section shouldn't break again.

You can, but there are a couple concerns.

You will definitely want to disconnect them from each other before charging - otherwise they may all end up connected to the same ground point and causing a short.

The picture looks like the cases may be metal. If so you need to check if ground inside is connected to the case. If it is, you need to make sure the cases don't touch each other,because you are hooking positive of one to ground of another, the cases touch would make a short.

If the 5v circuitry draws any significant amount of power, its battery will drain much faster than the others, so it may die long before the other packs are empty. Those packs may even cut off entirely when their internal battery gets too low, so one pack getting too drained will shut everything off.