If you configured MultiWorld with multiple UWorld instances to tick, then they are ticked sequentially.
The reason is that the threading model of Unreal Engine has strong requirements, and all the engine is strictly built around them. For example: the Game Thread is unique for the process (and for most platforms it must be the process main thread), there's one Render thread and one RHI thread, there's one Audio thread and, depending on the configurations, one Physics thread, then the UE Task Graph is configured to run on the remaining machine cores. Then the engine has predefined synchronisation points to make the systems communicate. All the Unreal Engine code is coded on the above assumptions, so it's not possible to tick worlds in parallel.
Please note however that Unreal Engine, thanks to the UE Task Graph, is already configured to use all the available cores when performing a single world tick. So adding additional threads could degrade the final performance.