There are other methods, but they don't necessarily keep the entire second tier in place--just the plate or board that it's on and possibly the bottom layer of the tier, so it leaves the top layer vulnerable to sliding off. At least with the center dowel skewered through all (or both), it helps to prevent possible sliding or separation of the layers under normal circumstances. Is there a reason you don't want to use a center dowel? (You could transport the tiers separated, if you aren't considering using one. If transporting stacked, you might consider inserting a few skewers into the top tier if there's a concern with slippage between layers.)