It depends on what kind of piece you are doing.

If you are playing a medieval piece, you can go up to a keyboard, stretch your fingers to play a fifth, then go crabwise up and down, playing fifths. You will soon learn why this is an alien (if not downright irritating) sound to modern ears. We just don't do this anymore.

If, on the other hand, you want to produce the 'bluegrass descant,' where the high voice finds the fifth of the scale and sails above the melody, then stay in the same key you started with. If the melody is in C, have the high singers stay in C.

Would you be interested in hearing a version of 'To Canaan's Land' that does that?