An agency put a lien on me and my husbands bank account for back child support. Is this legal since the debt is my husbands but both of our names are on the account?

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

Yes this is legal. As long as he is listed as a joint owner on the account then it is subject to seizure for debt.

Why doesn't you husband want to support his own child(ren)?
They can put a lien on anything you own jointly. That might also include YOUR income, if any, since under the law it is considered community property in most states.
Put yourself in his ex-wife's shoes, if you can.