Unfortunately the Dock will be limited to charging one system at a time. Per Dell-Cares:  The Dock is working as designed. Only the main cable is capable of charging the systems. The front USB C port only delivers an output of a max 7W and the rear USB-Type- C a max of 15W. Please refer to the manual for these specs. 
Hi, I have two laptops XPS 15 9510 and Latitude 5411 connected to WD19TB dock. I try to charge XPS using the second Thunderbolt (on the back of dock) using Thunderbolt 4 cable. Unfortunately, when I plug in I get message that power supply is to weak to charge battery. Is it possible to charge battery using the second Thunderbolt or it won't work? Main cable I use to power Latitude laptop and it works fine. 
XPS 15 9510, WD19TB and Latitude 5411