Hi JayDee;
I've been asking that question my self for years!  I ask a collegue of
mine a number of years ago and he said the combination may be helpful
at night when cough suppression would be beneficial to sleep and the
expectorant would simultaneously work to liquify the mucus.  Hopefully
by the morning one good cough and all would be well... provided you
don't choke during the night.

I really shouldn't be this cynical;)