I believe you go to "Settings > Downloads > Steam Library Folders > Add Library Folder", unless they changed it since I last checked. Then, you can select which library folder you want to install to when you go to download a game. Or for a game that's already installed to one drive, you can move it to the other by going to the game's "Properties" menu in Steam, then "Local Files > Move Install Folder" and selecting the library folder you want.
You can of course install games to both drives, as it shouldn't significantly affect performance having games on the same SSD as your OS.