how to let not visible control not occupy UI space?

Hi All,

I have 2 text input boxs need to be displayed conditionally, but the displayed place on UI should be the same, it means there is no extra space on UI

when condition A, text box A should be displayed, and text box B should not be displayed; vice versa.

I had used Visible = true/false to control, but the problem is even B is not visible but could see there is space for B, same as control A, even A is not visible, still can tell the space for A on UI. How can I only see A/B at one time without any pre-occupied space?

Thank you in advance,

Ellen