Given the information, with no other deductions/etc. listed, I would have to assume just the SD for you both which, if $28k is what you again believe will be your combined income, then your "taxable income" ($28k - SD MFJ $24.8k = ) will be $3,200.
For MFJ, with NO other information made available to me, you'd only be liable for SS and Medicare taxes, which will total approximately $765. Filling out "Exempt" on your W4 would not take out for this, and then you would owe. An option for you, that I HIGHLY suggest, is to fill out "exempt', then in the additional withholding amount line item at the near bottom right, ask for $35 per pay check to be withheld from ONLY ONE W4 (not both of you) then at the end of the year you will be covered for your FICA (SS & Medi) tax liability.
Your Income Taxes Breakdown
Tax Type Marginal Tax Rate Effective Tax Rate 2020 Taxes*
Federal 0.00% 0.00% $ -0-
FICA 7.65% 7.65% $765
State 0.00% 0.00% $ -0-
Local 0.00% 0.00% $ -0-
Total 7.65% $765
Income After Taxes $9,235
Retirement Contributions $0
