If the debt to the IRS was from before the two of you got married, then you have a good chance of protecting yourself from his debt. Hopefully your husband had already set up somesort of payment agreement with the IRS before you got married. If he hasn't do so already, he can make arrangements with the local IRS office, the sooner the better, you don't need to wait until next tax season to handle back taxes from prior years... remember that debt is still accumulating interest.
Correct me if I'm wrong, but I was pretty sure that a 401k is established in one person's name as the primary reciever, the other names were beneficiaries that would recieve your retirement pay only in the event of your death.
I didn't think it was possible to have a "Joint-401k" since your husband works for a different company than you do, and even if you did work in the same place, the company has to treat him as a seperate individual employee to properly account for pay, leave, taxes, and benefits (like a 401k) relevant to the time of employement and type of work. So... since the 401k is supposed to be in your name (he's just on it as beneficiary) and you're not dead, the IRS should not be touching it.
If for some reason it happens to be some wierd "Joint-401k" (which I doubt) where they're going to cut a SINGLE retirement check with BOTH your names on it, then you need change it to just your name (as owner and him reduced to beneficiary) other wise the IRS can tap that joint retirement account.
Keep your savings in a seperate account under your name only, make sure your husband's name is nowhere on it. Though it would be nice to think that the IRS would respect community property laws and only take half of whatever you have in it, for the most part they will seize everything in the joint account along with any individual accounts with your husband's name on it.
When it comes to filing taxes you have 2 options to try and protect your portion of any refund you may be due, neither are particulary appealing.
*Income Tax Withholdings
Before going into these options I advise that both you and your husband change the number of exemptions to "0" on your W-4 and state withholdings for all of your jobs. I cover the reasons for this in one of my previous answers to another question.
This is vital in your case, especially if you decide to go with option 2 when it comes time to file, since changing to "0" exemptions can help minimize the amount you specifically may owe to the IRS based on your individual income. Plus, in your husband's case, it will help pay off the IRS debt
*Option 1) MFJ and Injured Spouse Form
File together under the Married Filing Jointly (MFJ) filing status and also file Form 8379 (Injured Spouse Form) with it.
You would be considered the "injured spouse" and this is what the form looks like. If you look at the second page, it list the 3 requierments you must meet in order to be considered an "Injured Spouse" on the joint return. Normally you have to meet all 3, though you only have to meet 1 of them if you live in a community property state, this is covered on the 2nd and 3rd pages.
Filing a Form 8379 does NOT guarantee that your portion of any refund will be protected. The IRS reviews your joint return and determines whether or not to approve the claim. Though, if your husband's IRS debt was establish before the two of you were married, there's a good chance they will approve the claim and leave your portion of the refund alone.
Be aware that if you owe taxes to begin with (not enough withholdings, self-employment (Schedule-C) income, gambling/lottery winnings, or some other unexpected source of income) filing as injured spouse is moot, and you are still responsible for paying the taxes owed on a joint return. The injured spouse form is only useful in cases where there is a refund involved on the joint return.
*Option 2) MFS for each
Your other option is for each of you to file under Married Filing Seperately (MFS) filing status. Since your incomes and credits are accounted for individually, you will automatically keep any refund you are due and the IRS won't touch it. Of course if you end up owing (because you didn't have enough withholdings) then you would still be resposible for paying what you specifically owe to the IRS.
*A NOTE ON CHILDREN
If you have children DO NOT use Option 2. You lose most of the child-related credits and EIC (if eligible) when filing under the MFS filing status so your best choice is Option 1.