Interesting question, the answer of which seems to vary from year to year.
Down in SE Texas (where I used to hunt for many years), some years the darks would roost separately from the snows, while in other years they would all sit together (which we called "salt & pepper).
If you really want an accurate answer to your question, drive around some fields and take a look at the roosting birds. If you see some Greater Canadas mixed up with Lesser Canadas, then you do the same. If you see them separate, then follow their lead. This strategy has worked very well for me and most hunters in the past.
Zachary