SSRS Scale Out Deployment
First of all you need a proper https certificate available and installed in both instances (i suppose you installed SSRS on both instances and i suppose also that you are using SSRS native mode).
Then open Reporting Services Configuration Manager,
click Connect at the “Reporting Services Configuration Connection” Screen;
click on Web Site URL in the left-hand pane window
In the “HTTPS Certificate” drop-down box select the certificate you installed.  You will see the Port text box pre-fill and the Report Server Web Services URL pre-fill.
Click Apply