In many of those cases, the "joint licensees" started out with TV and then added radio later.This is one of the reasons I appreciate the Radio Discussions site: Once again, I've learned something here. Where I currently live, the NPR/PBS stations use the same calls and share a website. Where I grew up and attended college, the PBS/NPR stations did the same, and the hosts from the NPR station often did VO and positioning for the PBS station and even hosted the pledge drives there. That in mind, I just always ass/u/me'd they were the same entity nationwide.
That's our case here: WXXI signed on as a TV-only facility in 1966, but didn't add WXXI-FM until. 1974. (Unusually, WXXI AM didn't join the family until 1984, and it's taken us nearly 40 years to get another FM facility to do a full news and talk service on FM.)
Even many of the statewide networks in the south and Midwest started with TV in the 60s and early 70s but didn't add full radio services for a few more years, until NPR began providing more programming. Remember that Morning Edition wasn't created until 1979, and there wasn't a full 24-hour newscast service until the 90s.