I currently run SD on an iPad and on a Nexus 5 phone which I stick on the side window for a "HUD". It isn't easy doing route changes etc. on the phone, so would it be feasible to link the two together in flight so I could make changes on the ipad and it would be reflected immediately on the phone?

You can do this with two iOS devices by taking advantage of their Airdrop technology. Going iOS to Android without the internet isn't doable with SkyDemon though.