Entering Notes

I have a song printed from a PDF. It is written for the standard SATB vocal parts. It will eventually be sung acapella. Right now I want to create a score using exactly these notes to be used as piano accompaniment for rehearsal.
That means I need to have both the Soprano and Alto parts in the treble clef and the Tenor and Bass parts in the bass clef. After I enter a few measures of the Soprano part (notes and rests) then go back to measure 1 and start to enter the Alto part, the notes for the Alto part erase the notes previously entered stuff for the Soprano part.
How do I enter multiple notes or rests for the same beat?