On the new W-4 there is no "1" option any longer ... you should BOTH choose the single/ MFS option and leave the rest of the form blank. In a couple of weeks you see that you will not have enough withheld then one of you should add an amount to line 4c ... say you will be 1200 short and you are paid monthly then you would enter $100 on that line ... this amount is added to what is already being withheld.
Of course you changed last year in march which means you were under withheld for the first 1/3 or 1/4 of the year ... so if you extrapolate the difference in the quarterly withholding and you will be ok going forward you really have nothing to change.
Of course you could waste a lot of time completing the 4 page worksheet for each of you if you don't use my shortcut.
