I believe if aPandora would be healed by an effect then there would be one flip and she and Candy would both be healed the amount the flip implies. They would not share the healing they would both heal the full amount.
For your second question if a Doppelganger copied aPandora's Destinies talent then any effect applied to the Doppleganger would then follow the talents wording and then applie it's self to aCandy. As aCandy now has an effect applied to her it would then triggure the Destinies talent which would then effect aPandora. Note it would work much the same if you coppied aCandy's destinies talent except the order would go Doppelganger, aPandora, aCandy.
Also note that as the Doppelganger is not mentioned in the talent if it copied the Destinies talent and then aPandora or aCandy had an effect applied to them it would not chain to the Doppelganger as the talent refers to aPandora and aCandy specifically.