I am so afraid of stacked cakes!  I always use separator plates and pillars that drive right through the cake and leave a space between tiers.  How do you put a tier directly on top of the tier underneath and remove your fingers?

AI use the spatula. I slide it under, slowly move my hand from underneath while holdin the cake in place with the spatula, then lower the spatula.

I leave the dowels on the lower tier raised up about an inch.  Then I sit the upper tier on the dowels, slide my hands out, and push down from the top to sink the dowels into the lower tier.  

Post #6 in this thread will help...

http://cakecentral.com/t/710889/any-tips-for-stacking-fingers-spatula-in-the-way
 

Quickly whip your fingers out and drop the cake! - PJ x

I know it might not be an option unless you're an octopus but I've used 4 hands! I enlisted the help of my daughter who holds the cake plate by the edges while I placed it and took out my hand. She did the final lowering and steadying onto the pillars. 

get the cake as close to the lower cake as possible then quickly drop it. Quick, simple, easy.