Like samme said, you can just put your input that transitions to sceneC in both sceneA and sceneB, and that should work alright.
You could also run a scene in parallel in the background to handle your scene-switching inputs, and use that scene to start and stop the other scenes, kind of like a ‘parent’ scene. If you use scene.launch(‘sceneName’) instead of scene.start(‘sceneName’), the scene that called scene.launch won’t automatically stop and the newly-launched scene will run in parallel with the first one.