The box with 24 red cards and 24 black cards has a higher probability of getting two cards of the same color. Let’s walk through each step.
Let’s say the first card you draw from each deck is a red Ace.
This means that in the deck with 12 reds and 12 blacks, there’s now 11 reds and 12 blacks. Therefore your odds of drawing another red are equal to 11/(11+12) or 11/23.
In the deck with 24 reds and 24 blacks, there would then be 23 reds and 24 blacks. Therefore your odds of drawing another red are equal to 23/(23+24) or 23/47.
Since 23/47 > 11/23, the second deck with more cards has a higher probability of getting the same two cards.