Boxes 18 through 20 on your W-2 indicate local taxes and withholdings. If your W-2 has multiple entries (other than the Total) for this section, then you need to enter multiple rows of information in that section. All of this is done on the same W-2 page since it is from the same employer.
If you are using a CD/downloaded version of TurboTax, then you enter the second row of state information underneath the first one.
If you are using TurboTax Online, click the blue + Add another locality underneath the locality information until you have entered all the rows you need.
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