Adding Piano to Existing Score

I typed in the SATB part of the score. I want to add the piano part, which is largely the same notes as the choral parts. I copy and pasted the soprano part to the treble clef piano. Then I copied the alto part and dropped it to the piano treble clef part. It replaced the previously pasted soprano part instead of adding to it. Has anyone done this before? How can I get both the soprano and alto parts from an open SATB score to the same staff of the piano score? What works best?