I'm not sure whether it's buggy or not? Caldwell's ability seems to be:

1) not working when he's deployed again after a Decoy.
Is this a balance decision?

2) randomly not spawning one of the cards when deployed. E.g. I deployed him between two Arbalists in one battle, and that worked. Next battle I tried to deploy between a Wagenburg and a Warwagon, - he only spawned one ofthem (the Wagenburg).
What gives?..

So I think it's pulling cards only from the deck, only if you have additional ones there (unlike some other cards which specifically create copies).
Not that it matters since he's not around for long. But another chance to improve written instructions.

Actually it's quite clearly written in the tooltip for the card. :) They have to be concise as space matters, but it does say exactly how it works; 'Play a copy of each adjacent unit *from your deck*.'

So, if a copy of those two cards exists *in your deck* then great, you can summon both out of it.
If only one of the two exists in your deck, then you only get to play one.

If it worked as you were imagining, the card tooltip may read, '*Create* a copy of each adjacent unit and play them.'