Help with repeater "update row"

Axure RP 9
,
2

You just need to update your rule in Update Rows of the left repeater. Right now, your rule is:
[[TargetItem.Target == Item.Selection]]

…which equates to: The value in the (target repeater) column named “Target” matches the value in the (source repeater) column named “Selection”.
…so, the second repeater (named “( R ) Selected”) would need to have values in its ‘Target’ column that match the first repeater --which would only happen if both repeaters were identical to start with.

Since you only have one row in the second repeater, you could change your rule to:
[[TargetItem.index == 1]]

…which equates to “update the first row”.