If you used two pieces of silver and alternated current in intervals, you would just get two blackened pieces, albeit they'd collect oxide at half the speed. There are some generator plans floating around the net which have this feature, supposedly as a way to eliminate silver wisps. I'm not convinced this makes sense unless you never stir.
Also I'm not 100% sure if this applies to silver, but having oxide on the cathode may be problematic and result in a less clean product. This happened with me and colloidal gold. Now I always keep the cathode spotless, whichever metal it is.