I would say it's unethical...
If law schools were remotely interested in keeping the system accurate (you pay money, you are nearly guaranteed to go there.) When they don't get acceptances out with a reasonable amount of time to visit and make a decision, when they don't give out all their scholarships before deadlines and when they don't release financial aid information until mid-summer, I vote for being well within your ethical boundaries to submit two deposits.
If schools want people to quit submitting two deposits, they should give us the information necessary to make a decision before the deposits are due. Move back the deposit date, move back the application deadlines or just move faster. Right now many schools are so "We'll tell you when we tell you", so I see nothing wrong with students paying extra money for the privilege of doing the same.