Interesting, have you tried this?
My way of thinking is... if an IS lens and a non-moving sensor result in a steady image, then introducing movement to the sensor should negate that effect.
Why should it? IS (body or lens) doesn't apply a fixed amount of correction regardless of actual movement. IS is intelligent enough to determine how much the image is moving and apply accurate correction to the movement. So if lens IS has fully corrected the moment, IBIS would not apply any further correction. However, if the lens has left any movement uncorrected, IBIS ought to correct it further.
A limitation might exist in the amount of time each system is programmed to be active, but if a manufacturer is using both systems, the programming ought to be properly synced. I'd suspect that using both might take too long for some uses, so the user should be able to switch one or the other out.