as an expert of both characters, there has to happens some sick crazy shit to make those two fight each other for real. Sparing? Sure. Rival battle kinda like Roger's and WB's kinda rival battle? Ofc. But bloodlust full of hate battle rather not, if there won't be anything to destroy picture of one another since Usopp idolizes Yasopp and Yasopp miss his family etc the proud father on the way to get milk complex there. They would rather go on the side or try to calm both teams if others would be mad crazy to kill each other.