The listpicker is there for demonstration/example purposes only - it could be a textbox, the value of which is then tested against in the first list
My example provides you with two lists with aligned indexes, the index 1 in the first list relates to index 1 of the second list.
When you select/find/enter an item from the first list (which has unique values) you are selecting by the value in the list, but you also get the index of that item
Because the indexes of the two lists are aligned, you can use the index of the found item in the first list to retrieve the value in the second list using the index from the first.
Thus in my example if you select abc in the first list you will get 123 from the second list - both have the index of 1
If you select ghi in the first list you will get 789 from the second list - both have the index of 3
If you do not "get" this, then i suggest you read up, read up on Lists, and do some tutorials: