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You've created a dual axis in your first image. Dual axes are two independent axes that are layered on top of each other. In the second image you've merely brought the same axis into the view a second time, thus creating another row with that data. I've illustrated it below with Superstore data.
1. No dual axis. I've got sales and profit over time in two different panes.
2. I've created a dual axis with sales and profit, which are now included in one pane. Please note, that I haven't synchronised the axes so it looks as if both measures had very similar values. But if you compare both y-axes you'll see that the sales axis goes up to $260,000 whereas the profit axis only reaches $40,000. You can synchronise a dual axis by right-clicking on either of the y-axes and selecting 'Synchronise axis'.
Please let me know if anything is unclear.