Do you have to use two posts to create a 90-degree corner or do you have a single post option that allows the cable to thread through at 90-degrees?

Hi yes, you just need to look at the Dimensions Guide on the railing. In your case you need the CityPost part CP-HCP-G-36 which can be found at: https://www.lowes.com/pd/CityPost-Deck-Corner-Post-36in-Base-Mount-Grey/5013235331 You can always call 855-GET-CITY as well with any questions on how the parts work together. Thanks!