> cross universes?
MWI is a local theory. Changes spread out at the speed of light, they just spread out (and back from) adjacent universes as well. https://www.hedweb.com/manworld.htm#local
> the universes are identical...
Think of a 3D volume of space with a plane wave travelling through it, such as a beam of coherent laser light. An alternative way of thinking about this is that each point emits a spherical wave and the plane wave is just constructive interference of a continuum of spherical waves.
In many worlds theories, you can think of each point as also spreading out into non-spatial dimensions that are directions towards other universes. Similarly, aggregate behaviour is the constructive interference of not just the points in "one" universe, but also the points in a neighbouring "volume" of universes.
If the neighbouring universes are identical, or nearly so, it's like the coherent laser scenario. If they're all different, it's more like white light -- it's unpredictable and "random", but otherwise it's still just ordinary light.
Very slight deviations would result in various interference effects, such as the double-slit scenario.