I've been baking/decorating cakes professionally for more than 20 years, and I'm familiar with those extra-deep Wilton pans -- in fact, love them! Are you using the 3" deeps or the 4" deeps? I've never had a problem with 8" cakes, but larger ones can be difficult. Here are a few hints:
Do you have an oven thermometer? I suspect your oven temp may be set a bit high.
I always bake larger/deeper cakes at 325 for a longer time; for a really large cake, like 18", I even go down to 300 degrees. It prevents the problem of "mush in the middle, burned around the edges". When you change the cake pan size, the recommended cooking time is completely useless. Watch for the sides of the cake to start to draw back from the pan, and stick a toothpick into the middle to test for done-ness.
I also suspect that you may have put in more batter than recommended for the pan. I am awful about doing this -- oh, it's just an extra cup of batter, and I don't want it to go to waste -- wrong. The cake simply won't get done, and you'll have a mess. Do you have the Wilton yearbook? On one of the back pages, there's a chart that lists all the Wilton cake pans, the #of cups of batter each holds, how many cups of icing required, how many people it serves, etc., etc., etc. I don't know about cake mixes fitting into the pans; I only make homemade cakes, which is why people are willing to pay an arm and a leg for my cakes!
Putting two thin 8" cakes together is a great idea, but then you do have the problem of the "hump" on the side where the two come together -- this is sometimes difficult for me.
When baking larger cakes, you can take an empty, clean soup can (or large fruit coctail can if you need something slightly bigger around) that's cut out on both ends (so that it's a tube) and put it in the middle of the cake pan BEFORE pouring in the batter. This gives you a doughnut-shaped cake with "more edge" and less middle, and the cake'll cook more easily. Of course, then you have to place a piece of thin cardboard (cut from a cereal box, perhaps) wrapped in wax paper over the hole before you can decorate. I have several sizes of cans that I've been using for years; I store them with my cake pans, and I wouldn't know how to bake an 18" cake without them.
This is off-topic, but avoid large, square cakes like the plague. They are soooo difficult to bake. I know, it doesn't hold up to common sense, but they are really, really difficult. I make them, but I charge way more than for a round cake.
Finally, you can buy decorating dummies (made of foam) for practice. They are wonderful because they're perfectly flat.