Look at your panel specification:
Solar panel output: 6.8V 1700mA
USB output: 5V 1200mA
USB version: 2.1
This means that not only the panel has voltage regulator inside, but most likely some kind of support for USB battery charging specification. Connecting these in parallel is not a good idea.
So, check your panels - do you have direct access to 6.8V mentioned in spec? If you do, then that is where you have to tap in and that output you can connect in parallel to get theoretical (highly unbelievable, though) 6.8V 3.4A
If you don't have access to 6.8V then next thing to do is measure actual USB output voltage on a single panel. If it abruptly drops from around 4.5~5V down to 0 in low sun then the chip inside is too smart for your application, no kind of booster will help you here. You need different solar panels with unregulated outputs to connect them in parallel. These panels usually generate over 6V in bright sun.
Now, as mentioned in many comments, you need to understand the difference between DC-DC converters. Booster modules boost voltage, while at the same time reduce current. They are not happy if you put more than something like Voutput - 1.0V on their input.
When solar panel output can change from say, 3V to 6V, the output voltage 5V fits right in the middle of it. Which means you need "Buck-Boost" module, the one that can boost when input below and buck when input above the output.
As a side note, your reference to the unanswered question riddled with conceptual errors and links to questionable components underscores the lack of understanding. I applaud your willingness to learn this stuff. I don't believe anybody will be able to help you much until you do learn the basics.
Here are some connection options for you. In the first you need to figure out your own wiring, but you'd keep an option to use USB output of a single panel, if necessary. In the second you can use USB cables that you already have, but you might accidentally damage USB device if you plug it directly into panel.