Blackmagic Design DaVinci Resolve Advanced Panel - Manual (Page 180)

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The Gang and Show checkboxes of VTR B are now enabled.
3. Turn on its Gang checkbox to display that playhead in the Viewer.
The accompanying Show checkbox automatically turns on, and a second frame
appears side by side next to the first one in the Viewer.
4. To select which clip appears in this new frame, turn on the Select checkbox of the VTR
you just ganged, and then click a different thumbnail in the Timeline.