I think it should be possible to output within the collection list module two or more collections. Why?
For example, you have two collections A and B which displays work out of your portfolio. You need two collections because A and B have totally different layouts and fields, but in your overview of your portfolio you want to display one list, that should be sorted. In the moment that's impossible with Webflow.
Below is a little visual, which shows the above-described use-case.