Congratulations on stepping up to a 16"! I just recently did my biggest project so far (14/12/10/8 in. cakes), and the largest cake was 14".

I used the following methods to end up with gorgeous, moist, 2" high cakes that frosted beautifully. I use doctored Duncan Hines cake mixes and get approximately 5 cups of batter per mix (1/2 cup more or less depending on the type of cake). [When I say "doctored", I do NOT mean using an extender recipe.]

Determine how many boxes of mix you'll need for each 2" high LAYER. (Each 4" high tier will have TWO 2" high layers.) Figure that each box mix will yield about 5 cups of batter, then look at this chart:

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

For the first of two total 16" layers you will need 15 cups of batter (or 3 box mixes). Mix 3 box mixes. Pour into 16" AFTER pan prep (see below), bake on middle shelf of oven at 325 degrees. It will take a long time.

AFTER that layer comes out of the oven to cool, THEN repeat above for the 2nd 16" layer.

If you have a KA that holds 4-1/2 to 5 quarts, you can mix two box mixes in your KA (double batch), then do the 3rd box in a bowl with a hand mixer. It will be ok to let them sit on the counter until you get the 3rd box mixed. Do NOT add them to the pan until all 3 are completely mixed.

PAN PREP: Methods to make sure you get a completely baked, 2" high cake that is level.

Bake Even Strips and Flower Nail on larger cakes --Photo tutorial:
http://www.wilton.com/forums/messageview.cfm?catid=8&threadid=148262&FTVAR_MSGDBTABLE=

TUTORIAL: How to collar a cake pan...
http://www.wilton.com/forums/messageview.cfm?catid=8&threadid=154355

I fill my pans a little more than half-way. I prefer to have some left-over batter (freeze or make cupcakes) than to have too-little batter. Using the "slight overfill" and the methods above, I was able to have two dead level, 2" high layers.