I have a 650 and dual G3X Touch. I purchase the Nav Data for the GTN for $299/yr and US G3X Touch Bundle for $149/yr. The G3X is honestly a nice to have if you don't want to use your iPad in flight. This allows me to use an updated approach plate (with overlay) in the G3X touch and be legal with the GTN driving the Navigation/approach. All the other "stuff" in the G3X bundle is very nice to have... Terrain, Obstacles, etc. But at a minimum to be legal you will need the 650 updated. Oh and to answer your question on the dual screens, you only need 1 subscription for both.