hi there nate... I think the function you're after is: Go TABSELECT=prev
And if you REALLY want to assign the function to <CTRL+TAB> then have a look at this topic from Icfu. It's a nice trick to get around the fact that the Dopus command/hotkey editor will not let you assign the key as a hotkey. Though technically I'm sure it's not 'supported' by GPSoft... it seems to get the job done. I've attached a zipped up .dpf file that after extracting you should be able to double-click on in order to import it and be good to go.
Edit note: I do believe I'll keep this and use it myself... 
CTRL+TAB=LastActiveLister.zip (421 Bytes)