Assuming it is the metal arm type mounting for the Dish and you would ike to use the same arm for the cell?
I could think up different ways to make a "stand-off type mount but I also might not do it.
Electronics have lots of weird ways to kill our fun and one is the way different frequencies can interfere with each other. So putting the cell antenna too close to the Dish may get into some really weird things that we can't see nor measure as we don't have the equipment for most of us to test.
Sometimes it seems harder but I often go for keeping my little doubts separated when I can as it lets me deal with one frustrating bug at a time.
My brother ran into getting interference on living room TV and finally found it happened when the LED accent lights were on in the bedroom!!
I don't go there if it can be avoided!