I think it would be a cute way to reveal to them.
We did a b2b so I chose Mickey for one and Ariel for the other. As pp says, they will say that they are looking forward to you joining them.
I would love to have surprised my kids this way, but alas, I can't keep it in and they know all of our vacation plans all the time. However, we did somewhat surprise them with the second part of the b2b cruise. I remember telling them that we would do a b2b when I first booked it, but they must have forgotten and when we were at port at the end of the first part, they said they were sad it was so short. So my husband and I realized they had forgotten. So on the morning before we had to disembark, we decided to ask them if they had fun and if they wish they could stay on the ship for another cruise. Their eyes went wide and said "SURE!" And when we told them that we can, they didn't believe us. We even told them that we had paid for the cruises a long time ago and that we can. Our luggage (we changed rooms) is being transported to another room. They still didn't believe it. When our server came over, we told her and that's when they finally believed it.
We enjoyed surprising them with this even though we didn't know it would be a surprise since they always know our vacation plans --Disney! every 6 mths. It hasn't changed for 7yrs. I like this type of surprise, because it didn't take any work from us. I think they really didn't know you can do b2b's.
Good Luck with yours.