So is the advantage of using two maps only so that you can present a subset of the survey (from the separate map) offset... In this case yes.
...or at a different scale or similar in the same output PDF? Cannot do different scales in the same output pdf as far as I know.
I've not yet got as far as working out how to combine multiple maps into one output document.
It's as easy as; select map1 select map2 select map3 in the thconfig file. The resulting pdf will then have a layer for each of the maps and for the surface image if you have one. These layers can be toggled visible or not by the user of the pdf viewer. Aside from that, I use maps hierarchically the same way as we tend to do with survey centrelines as a basic means of managing a project. Each section of cave has its own map. These maps are in turn collected into maps, then these are in turn collected into a map of the whole cave. Then perhaps a map with several nearby caves and surface features. You can choose to export the map at the top of the 'tree' in which case you do not get a layered pdf, or you can export a selection of maps below the top of the tree to get a layered pdf. I've tried to explain my understanding of it, somewhat unfinished, here http://therion.speleo.sk/wiki/doku.php?id=bds#concepts Bruce