You don't add anything together.  Enter each W2 separately and the state is in box 15-17.  Don't enter the state withholding again anywhere else.  Only enter anything 1 time.  

Yes if you paid state tax due last year you can add those together for a federal deduction if you itemize deductions.  But most people are now taking the Standard Deduction since it has increased.  

You NEVER combine income or tax paid in different years on the same tax return.  If you have more than one W-2 for the SAME tax year, you enter those W-2's one at a time in Federal>Wages & Income.  After the first one, you click  Add W-2 to enter the next one, until every W-2 for that tax year has been entered.

As for entering state tax paid---exactly what are you asking about?   Do you mean you paid  state tax due when you filed your previous year state return?

  If you paid taxes due in 2019 to your state or local government, you can enter that by going to Federal> Deductions and Credits> Estimates and Other Taxes Paid> Other Income Taxes.  You can enter the taxes you paid, but do not include any penalties or interest.

Where are you seeing the tax consultant added them together?  What line on your tax return?  You just go though the interview steps and  enter everything individually into Turbo Tax and TT adds them together and  puts them on the right lines.   If you got more than 1 W2 for the same year....after you enter the first one you can add another.  

All state taxes paid during the tax year are totaled on the Sch A automatically by the program if you enter these items in the interview screens correctly ... make sure you enter them in their own places ... do not rush and add them together and force the total ... let the program do the math for you as that is what the program does. 

Thanks a lot for all your responses. I realized that my question was not clear. So I have corrected it as "I worked in 2 different companies last year with 2 W2s.  I understand that the state taxes that I paid for those two (from my 2 W2s) would be added for State and Local taxes. My tax consultant has also added my state tax paid in 2018 to State and Local taxes. Is that correct? I am trying to move to TurboTax and trying to understand how to do this with TT. Does Turbo tax add the previous year's State & Local taxes automatically or should I enter that manually if I file through TT instead of Tax consultant ? I am trying to move to TurboTax and hence my question. Thank you"