You should be able to move UIBB(2) under Subview 1 like this:
Main View: View 1
Subview: Tab1
UIBB: (1) ZTEST_OIF WINDOW1
UIBB: (2) ZTEST_OIF WINDOW2
Then, delete "Subview: Tab2". If a tabed component has only one tab, it is displayed in a single frame. In order to add the collapsable panes, you need to select the appropriate attribute for each UIBB.