If you use the worksheets on the W-4, the instructions say to fill out one worksheet for both of you, including your combined income and deductions, and then split the allowances between you. (If the worksheet says to claim 4 allowances, you can claim 0 and 4, or 2 and 2, but not 4 and 4). The instructions also say that in most cases, the withholding will be more accurate if all the allowances go to the higher earning taxpayer and the lower earning taxpayer claims single with zero.
The IRS calculator is also very good but you won't be able to use it to adjust your 2019 withholding until after your first paychecks in January 2019. (If you used it now, it would tell you only about the last pay period of 2018.)
