In my scrap parts box, I ran across a good-sized two-gang variable
capacitor that I hadn't seen in years. It has a lug off of each gang.
I was wondering if I could convert this to a simple antenna tuner in
some way.

There are a LOT of informative sites regarding antenna tuner projects and
experimentation, and a search on those topics alone should hit more than you
can read in an evening... as for using both sections or just one, it depends on
a few factors, which have mostly to do with the frequency range desired, and
the size and type of antenna you're wanting to tune with the capacitor. Again,
a search for antenna tuning projects and tips should garner you a hefty amount
of info. You'll love it!
Have fun-
Linus