The question then would be, "why" can't you combine more than one IN_TREE clause? Is this some hard-coded implementation of Alfresco that limits this? Because conceptually, it feels like one should be able to do something like this, since content items in CMIS are allowed to be multi-filed, and by combinding two IN_TREE clauses you could filter results to find only those that actually are multi-filed.
I have a GUI in which the user selects a list of sites for downloading documents. I can not get all the documents from the selected sites in one request? For each site will have to form a separate request?