Do people ever have more than one agent, if they are writing in different genres? What if they are only a little different like the difference between pictures books and middle grade?
Generally you get one agent per writer.
Some writers have two, but only if the categories are quite dissimilar. The most frequent example is fiction and non-fiction, followed by adult and kids.
It's not likely you'd have two agents for middle grade and picture books.
It's hard enough to get one agent.
Try not to worry about whether you need two until you have one.