Running Headers: Dictionary style showing range

I am working with an Index. What I want at the top of the page is to show the text from the first GroupTitlesIX on the page followed by a dash and the text of the last GroupTitleIX on the page. For example, if the first GroupTitlesIX on the page is A and the last one is E, I want to see A–E at the top of the page.

Unfortunately, what I'm getting is A–

The master page is set up using two different running header variables:

Running H/F 10–Running H/F 11

Running H/F 10 is defined as <$paratext[GroupTitlesIX]>

Running H/F 11 is defined as <$paratext[+GroupTitlesIX]>

Am I missing something, or is this just not possible to do?