I receive social security benefits. For some time now the IRS has been levying 15% of my monthly benefit amount. My understanding is that this is legitimate. However, this month the FDIC has also begun levying an additional 15% of my benefits. (I have an old amount due the FDIC from '92. My question is can they levy 30% of my benefits or must each one take a part of the 15%?

I assume that your Social Security benefits are NOT your only source of income. If they were, neither the IRS nor the FDIC could levy ANY of your benefits, because doing so could impoverish you.

So, if you have other assets, then the FDIC garnishment is perfectly legal In my opinion.