Hi Robin, I don’t think this is documented that extensively. The feature is called MultiWACZ, where multiple WACZ files are loaded from a JSON file. See for example this issue pointing to Continued problems with warcit and wacz - #4 by Hank.
For a starting point reading the source code, see wabac.js’s loader.
I don’t think MultiWACZ has been integrated in the WACZ spec yet, but that seems to be in progress. Note that browsertrix(-cloud) uses this already, so it is something that should work.