Is it unethical/illegal to send in seat deposits to two different schools, if only to buy more time on making the decision. I realize that they are non-refundable, but if I can't make up my mind it is probably worth the few hundred bucks to have another month or two to decide.
Are there any other consequences from doing this? Can a school take away your spot if they find out? Do they care?
This is unethical and selfish. Don't do it. Do you really think your difficulty in deciding is going to magically disappear if you give it just a few more weeks? What's your endgame here? You need to think seriously about why you can't make up your mind now. What is going to change between now and two months from now? I realize this is a tough position and I sympathise with you. Perhaps instead of cheating your way to more time, you can approach the schools and ask for an extension to the deadline. Or maybe you just need to bite the bullet and make a decision. If your on the fence about it, the odds are that neither school will be a bad choice.
Think of it this way: You are lying to the school and making a contract with an intention to breach it. I'd say that's wrong. What makes it even more wrong is the burdens that you place on other people. If you put down a seat deposit without any intention of going there, you're screwing over someone who really wants to go to the school and is sitting on the waitlist. You're also adding tons of extra work for faculty and staff who would begin preparing for your arrival.
In terms of consequences, I'm not sure what would happen. Schools could of course find out. I'd hope that they'd deal seriously with something like this. The legal profession should be open only to those with honesty and integrity.