In my opinion, the best way to handle any theoretical future exploration multicrew sessions (with Frontier's above design criteria in mind) is to have every commander on board share exploration data equally, with the single caveat that only the ship owner gets first discovery tags.
As I understand it having a second human man your turrets or fly your SLF can make you more effective as a combat vessel and so means you make more money and therefore it is only right that the other player gets a cut. There would have to be a mechanic that allowed two players in the same ship to make exploration money more effectively than one on their own to justify credits for the passenger, IMO.
I don't know what that would be but unless it exists giving the passenger exploration data would just feel wrong to me.