What would prevent you from using both?
NOTHING! Thats what I do...I also have .a givi tail trunk (scooter size) and I mounted it to a spare passenger seat.
For a tankbag, I use a magnetic one. The Joe Rocket Manta xl. I also have a Manta, but the xl version is better. Only slightly larger, but a bit more capacity.
When I first bought it, I used it occasionally....Then, you get to where you cant live without it. If you worry about theft, its easy to pull it off the bag and bring it in the store.
I carried a watermelon and a gallon of milk the other day. Its also carried human ashes....
It works on all bikes with metal tanks. If your worried about scratches, drop some paint protection film down on the tank before using it.
Oraguard is good. 3m scotchguard is good. Cheap on ebay.
I use it 10 times for everytime I use the rear bag.