My plan seems to have backfired. Actually, only David is washed onto the tidal islet of Earraid. Alan is washed ashore with a group of sailors onto the mainland of Ross of Mull. When David finally learns the secret of the tidal escape from the laughing fisherman after four days of being stranded, Alan has preceded him on Ross by several days and has left a message behind for David to join him in Torrosay. I posted this because I found 2 or 3 questions in the Kidnapped Question queue that all asked "What was it like for Alan and David for the day that they were stranded on Earraid?" I thought a nice discussion from other points of view would lend added clarity for the question askers (and composer). Didn't mean for it to be a trick question! Honest I didn't!
It has been a while, but don't both David and Alan get washed ashore onto Erraid? Of course, David is stranded their by himself, whereas Alan and how ever many surviving crew there were row onto the island. David spends a few days wandering around until he meets up with Alan again.
I have to agree with the previous poster that both of them actually get washed ashore on the island. I think it is a also a few days before the two characters end up meeting together. David was on the island first then came along Alan.