Simultaneous Long and short notes

I am composing a solo double bass piece and want to write a minim low C drone over which there is a quarter note melody. What I am finding is that the default position is that when I add a minim the wanted low note takes on the value of the quarter note when it is added under the melody line. If I add a minim first and then add the melody line afterwards quarter notes that I want grab the time value of the minim. So the question I have is how to go about separating out a minim with quarter notes in the same stave? Hope someone can help!