It's unlikely we'll deviate from our preferred system and it's unlikely we'll start many games, if any, with them both up top.
However I don't go along with the suggestion or belief they are incapable of playing alongside each other. In certain games it could work. They're both very intelligent players. They would need to adapt but it could work. Also since getting more game time, a full preseason and up to speed Defoe is showing the great player he is. Morelos has also taken his game to another level.
For it to work it would take time, not too dissimilar to trying to get Defoe and Davis up to speed last season which didn't work out too great. That's the big risk. It's certainly not the worst problem to have. Thankfully it's all on Gerrard! As above though I can't see it happening too often.