I married this year. My spouse has unpredictable income. What should I withhold on new W-2s? He does contract work. I work a 2nd job. File jointly or separately?

Joint is usually the best way to file.  Here's some general info on getting married.  Did you both use Turbo Tax last year?

You can not transfer from or combine 2 returns or accounts.  Just pick the return for the spouse that has the most complicated return to enter or has any depreciation or business or  more investments.  Or use the account for whoever you want to be listed first going forward (by the way you can not change the order of the names).  Then add the other spouse's name, ssn and info to it.

You should usually  file a Joint return which is only 1 return combined for the both of you.  Make sure to stick with this one account for the future.  Just ignore the other spouse's account.

Getting Married FAQ

https://ttlc.intuit.com/community/tax-filing-status/help/what-does-getting-married-mean-for-my-taxes/00/1756642

And info on Getting Married…..

http://turbotax.intuit.com/tax-tools/tax-tips/Family/Getting-Married/INF12006.html

Is it better to file Joint or Separately?

https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separately

Compare Joint to Married Filing Separately

https://ttlc.intuit.com/questions/2895312-how-can-we-compare-married-filing-jointly-with-married-filing-separately

Your employer should include your tips in the wages reported on your W-2, and withhold tax on your total wages, including the tips. Having tax withheld on the actual amount of tips each pay period should take care of the fact that the amount is variable.

If your husband's income is unpredictable there's really no way to figure out in advance how much tax you need to pay to cover it. He will have to see how much he has made at the end of each estimated tax period, and then make an appropriate estimated tax payment for that period.

Theoretically you could attempt to adjust your withholding periodically to cover the tax on the income that you know he has earned so far, but it's not really practical. You would have to submit a new W-4 to your employer every few months, and it would get very tricky at the end of the year. You can make an estimated tax payment in January to cover the last few months of the year that just ended, but withholding has to be done before the end of the year. Tax that's withheld in January can't be applied to the previous year.

As VolvoGirl said, you will almost certainly pay less tax if you file jointly.