Good point Stikki. The athlete’s 400m has to be seriously quick to be able to go through the first 400m in sub 49s. The accepted buffer is 4sec so Rudisha is probably in low 45s shape.
In Australia there are many schools of thought who still think insane levels of mileage is the key to 800’s and refuse to acknowledge that getting the 400m time down is of any importance. They expect 50s 400m runners to be running sub 1-50. Given the expected buffer of 4s this has the runners going through the 400m mark at 54s - hardly a recipe for fast 800’s.
Unless the runner can run reduce their 400m time to a serious level (at least sub 47s), they will never be an elite 800m runner.