Make A furnace have random outputs

So, i have a block called Obsidian refiner, that works kinda like a furnace, once ou smelt the obsidian, there is 3 output blocks : 

i want that for each slot (03 04 and 02) there is a 50/50 chance of having obsidian dust, and 50/50 chance to get obsidian shard, how can i make that, for now i have no procedures for this furnace