Ok, so I found the 'Save All Listers' option which does what I need, but not entirely intuitive. At first glance, your impression is that it does just what it says.. saves all the listers, not just the one with the current mouse focus. But no, it does something else. It saves all the open listers under a single layout [to be opened together later with one click]. 'Save ALL As One..' would have been more helpful.
Then there is the opening of the new combined lister. There isn't a command or option to do that. Not a [File][Open]. Not a [Settings][Lister Layouts][Open]. No, the only way to open your new layout is to find it in a recents list, which quickly grew to 3 entries (the third being a grouping - or 'combination' - of the previous two). I suppose you could use Agent Ransack or some other file search tool to crawl your PC for the location Dopus stores these layouts, but that shouldn't be necessary for a tool of this age and maturity.
Let me see if this fixes my problem. Maybe there's a new paradym in software design that I've missed since my last DO experience. Note that I came back in 2015 and don't recall having these issues.