Thank you Jerry1991 for your answer.

I have now connected a meter between the socket and the charger and was able to measure the power consumption when charging the powerbank:
Charger 3 watts and one with 16 watts. Attached individually, the powerbank draws the corresponding values. If both are connected at the same time, only 16 watts are drawn, i.e. not 19 watts. The power bank is therefore only ever charged via one channel, but always the one with the highest value. In this example, 16 watts. It doesn't matter in which sequence I have connected the cables.

Important:
- You write that the Powerbank charges very slowly. Make sure that you use a charger with at least 25 watts. I have a charger with 45 watts, but it only drew a maximum of around 25 watts. It took me about 2 hours to go from 50% to 100% charge. Assume that a full charge takes around 4 hours.

- I had used a "wrong" 10 watt charger at the beginning. However, it then only charged with 3 watts (for whatever reason). It then takes 33 hours to charge ..... I thought the powerbank had a deep discharge problem. Greetings

Hi. So I saw that too. I just tried it individually. Works on both sockets. Then I connected both. The unit also charges in this way. But whether that is twice as fast (??) I can't tell you. It generally charges very slowly for me. But I will continue to observe this. Greetings