Solution1: use pathing report in Analysis workspace, use initial value as account registration page and then see traffic that go to Sign in or Register screen if they are 2 distinct screens
Solution2: Fallout report, in the same spirit as pathing use first item as account registration and do one fallout for sign in and one for registration
Solution 3: use segment where in visit there a HIT with PAGE A and THEN without one hit PAGE B
Solution4: If the buttons have custom link tracking implemented pull data for these value in Adobe Analytics for the custom link dimension
You can also read this : https://www.fullstackanalyst.io/blog/adobe-analytics/using-flow-and-fallout-visualizations-like-a-ro...