How to make button-activated sliding panels that disappear behind white line

Hi! I'm trying (really hard) to make a row of panels that, when their corresponding button is pressed, one panel slides down and disappears behind a white line, while another, hidden panel, slides up, also behind that white line. In other words, clicking the buttons make two panels switch in a toaster-like fashion. I'd also like to make the button have a binary function, i.e., one click switches them, another click switches them back. Please help me, it's so confusing (see ID file attached and check EPUB preview panel to get an idea what I want). Thanks!