In the previous release, it was possible to do a Saturn V style "dual plane separation" by placing an upside-down structural tube to an engine plate node and using the plate's 'jettison' action (using a right-side up tube or decoupler would stop the engines from producing thrust).
This still works exactly the same way except in 0.1.1.0 the jettison doesn't actually separate the tube from the engine plate and it just floats there.