I suspect you said it yourself - 'they are not strictly related'. This isn't a matter of protocol or anything silly - it's a question of what sort of diagram you've asked for.
I've got a similar set-up - my 4G GM had 2 husbands. If I ask for a Descendants Diagram based on her, it shows her 2 spouses, plus any descendants of her 2 marriages. As the 1st wife of her second husband isn't GM's descendant (or a spouse who could give her descendants), she doesn't appear. Even an 'All Relatives' diagram won't show Wife1 of Husband2 as she isn't a relative by blood or directly by marriage.
You could have a look at an article in the Knowledge Base on 'View Both Spouses Ancestors'
http://www.fhug.org.uk/wiki/doku.php?id ... _ancestors
Essentially, in the 1st method, you select your mother, hold down the Ctrl key, select her 2nd spouse and play around with the diagram types until you find something that looks useful. I found a All Relatives diagram seemed to work, though it only gave All Relatives of the 2 selected people and descendants of the 1st spouse - because they are descendants of her.
The way I usually do it, is to get one tree shown in the diagram, then do menu option Diagram/Inset Into Diagram, which allows you to insert a new tree entirely, so you have one diagram with two trees. You'll see ribbons connecting the common people (err - that's people common to both trees, I mean!).
Essentially you need to remember that not everything you can envisage can be easily programmed - usually because you think of the simple case but the software has to cope with all possibilities, and sometimes it just doesn't work for all possibilities.