I was kind of wondering the same thing but thought maybe I was approaching this all wrong. I think the first tray would have to be so packed with vermicompost to present a more natural escape route to the new tray placed on top. Not sure if that's the way to make healthy compost. Because my worm population has grown, I'm currently working four trays and the top one is always the driest while the bottom tray is always the wettest. So I rotate the tray positions every week when I add scraps.
I guess if you think of the worm compost bin like you would an outdoor compost pile, it would make sense about the initial load of raw compost material - the initial breakdown of that material to get it to the preferred state. After the first year of religiously using coir (hard to find locally), I've used just newspaper unless I happen to have the rarely-used coir on hand. And I add finely ground eggshells for a little grit. Some folks use some garden soil but I would think it would have to be pesticide/herbicide-free.
