Well, you would need to post some code for us to see exactly what is
going wrong.

However, I would start by architecting the solution this way: UC(A)
declares a public event (which you did). UC(B) exposes a public get/
set property for the ID that it needs in order to show what it needs
to show. (If it needs more than an ID, such as say a row in a DataSet,
it could expose a property to accept the entire row.)

Then, in your enclosing form, subscribe to UC(A)'s event, and in the
event handling method get the ID (or whatever information) from UC(A)
via some public property that you define, and then use UC(B)'s public
property to set the info in UC(B). In other words, the enclosing form
acts as the go-between: it reacts to the event from UC(A) and then
feeds UC(B) whatever information it needs.

This way neither user control needs to know about the other. It's a
bit more code to write, but it's way easier to debug.