We are filing a joint return. Can we add together our gross incomes,? our taxable incomes? our income tax withheld?

You enter each W2 and 1099 separately.   They will be totaled on the summary.  Be sure to pick the right spouse when you enter each form.  After you enter the first W2 or 1099 you can add another.   

When you are entering income on a joint return, every income entry screen will either have a spot with both spouse’s names there, or when you click to enter a W-2, 1099R, etc. it will ask you by name which spouse it is for.  You do not have to wait for the software to “ask” you for your spouse’s income. Go to the screens to enter your spouse’s W-2’s, 1099’s, etc. just like you did for your own income.  Then look for your spouse’s name there.

If you mean, since you are filing jointly, is your tax situation such that it is based on the totals for you both, yes, your return is calculated on the total of you both, so it would be that same as if only one of you earned all the gross income, all the taxable income and had all the tax withheld. 

Yes, the tax won't be different if you add everything together to compare this year to last or to make a budget for next year. 

Did you mean something different? 

@rothll