I would bring them just in case. When I went last summer we went to AK for morning EMH and it was our first time and my son was so excited that as soon as the person in front of him put their ticket in and began to wak through that he put his card through not realizing that the person in front of him had a problem with his ticket and he was small enough that he would just duck under the turnstile b/c it was too hard for him to push on it. So off he went to meet up with my friend who was waiting for me and DDs to enter. Well the lady started screaming for my son to come back and he didn't realize she was talking to him so he just stayed there - she was making such a fuss she didn't hear me say what's the matter (somebody cut in front of me last minute & didn't speak English when I said excuse me, but I was able to keep my eye on him so it wasn't the end of the world) I was able to get her attention and said that he was my son and what's the matter? And so very rudely (and very unDisney like) she explained what happened and then demanded that I show her mine and my son's ID cards or she would call security. I showed her my ID card and politely said, "I'm sorry I don't have my son's card, it is with my huband in his wallet and he missed the bus b/c he had to run back to the room b/c he forgot his camera, so he's on another bus." She started screaming at me that I was abusing the privilege of EMH and how they were for guests only and I was taking space from another person and that I was rude and thoughtless, etc, etc. Now the line behind me is growing and growing and people are getting angry (not at me thankfully), but I politely said, "Ma'am, if you look at my ID you will see that we are resort guests and are not abusing EMHs - the dates are here, I just don't have my son's card it is my my husband's wallet." Another CM came over, whom I guess was security b/c he had a walkie talking and came from the security office - she told him what happened and then said that I should be removed from the park b/c I was trying to abuse the system - I showed him my ID and explained that DH had the ID and was waiting for another bus and would be happy to bring the ID to security when DH got there. The security guard looked at her and politely told her that she was wrong and I wasn't abusing the system and there was no wrong doing whatsoever. The CM apologized to me profusely and said I could go and returning was not necessary.
Sry so long winded - just make sure you do have the IDs and if one person has to stay behind for whatever reason, make sure the other person has the necessary IDs