Blue Sentinel

Q. Do I have to pay for each trigger being doubled? For instance can I exhaust Rek'Sai, Void Burrower to get double the reveal effect?

You must pay costs at finalization. Red Brambleback/Blue Sentinel would indeed double the Conquest trigger of Rek'Sai (becoming pending items), but you would only be able to pay and finalize one of them onto the chain (as you can't exhaust twice). Effect will therefore resolve only once.

Q: Do their "Conquer/Hold" effects apply to both units and the battlefield?

Yes.