Guess how you play a fugue on a non-polyphonic instrument.
Maybe if you hiccup while playing, two notes will pop out at the same time and there is your fugue. Yes? No?
How about you play one part and you just think the other part in you mind. Yes? No?
Ok, I know, forget it....
Well, given that you can't play two voices at the same time, you use time division multiplexing, which means you divide up time to let the voices take turns. Give a few notes of phrase fragment to one voice, then a few to the other, then back again, and so on, leaping back and forth between the two voices. The trick is to keep the voices well-separated in pitch (typically by more than an octave), so that a small leap (a small change in pitch) means you are moving to the next note in the same voice, but a large leap means you've jumped across to the other voice. That way the listener can tell which voice is playing at any given moment.