iPad: Battery usage while using split screen vs single app in foreground
It is not possible to give any kind of “always true”, generic answer, as the power consumption behavior is different from app to app, and even depending on the input data in each app.

In general the power usage by each app is not at all limited to only CPU usage when taking the display itself out of the question. Other major power usages comes from the GPS, cell radios, WiFi radios, the GPU, etc.

However if the app in split screen is doing the exact same stuff as when it is in full screen, then yes, the power consumption is approximately the same as the sum of the power consumption of each app by itself in full screen (when not taking the display into account).

In practice the split screen app is usually doing very slightly less, so the total is often slightly less than the sum. However the difference is negligible.

Again - this can vary hugely from app to app.