Even JMS has actually admitted years ago that DS9 and B5 being in development at the same time was a coincidence. DS9 was already in early pre-production when he pitched B5 to Paramount. They turned him down, but also asked if he would be willing to retool it as a Trek show. Most likely, they hoped to recruit him as a DS9 writer and didn't expect him to turn that down. Granted, it seemed super suspicious, but now that Paramount's documentation has been released it's pretty clear nothing nefarious happened. The DS9 showrunners weren't even informed the B5 pitch had happened and the network didn't interfere with DS9 much if at all, so the only cross contamination came from the handful of writers they shared, like D. C. Fontana.