I was a resident in two states (WI/PA) this year. Where in TurboTax do I indicate the state I lived in when I received 1099-INT income to file on the right state return?

As you are preparing your state income tax returns, you will be able to allocate income to each state. Click on How do I allocate (split) income for a part-year state return?

I recreated your situation in TurboTax. 

On the PA return:

1. on the Adjust Federal Income Interest screen, answer Yes to adjust the interest amount.
2. on the next Adjust Federal Income Interest screen, enter the amount not received in PA in the Adjustment Amount box and click the circle next to Other adjustment. 

On the WI return, on the Let us know if you made any non-wage income outside of Wisconsin while you were a nonresident screen, enter the Non Wisconsin Income amount.

[Edited 1-28-18|12:03 pm PST]

When I go there, it shows my W2 income, but not my 1099-INT income

See my revised answer.