You are 99% correct. Both stations shared time with others until they "got together" in 1929. WENR had been on 1130 to start (1925), then 1040 (1926-28), while WLS was on 870 from Day One in 1924.
Some NBC Blue/ABC programs also aired on WCFL until ABC bought WLS and merged it with WENR in 1954. WLS was not a full-time affiliate, even during the time they were on the air. Prairie Farmer programming took priority, and WLS owned most of the airtime.