You can create a set to solve this problem. Right click on customers in the dimensions pane, select Create > Set
The screenshot below captures the rest of the settings. The key is selecting Order Quantity as the field, and the existing top N parameter.
Adding this set to the filter shelf will show the same top N customers on both pages of your dashboard.
You may also want to sort both worksheets on the dashboard in the same order. To do this, drag the Order Quantity measure onto the rows shelf in the Profit worksheet. Click on the pill and change it to be discrete (blue).
Then add a Rank table calc (click on the blue Order Quantity pill) that looks like this:
Lastly, drag the discrete order quantity pill in front of the customer pill. You'll also want to hide this column, by clicking on the pill and deselecting "show header"