What benefit was gained by splitting their forces between two land branches instead of just conscripting everyone into one?

Basically none. The desicion not to integrate the Waffen-SS into the Heer was a political one, not a military one. The argument was that the SS were intended to transition into a kind of elite polcie force once the war was over and therefore should not be a part of the army. In practice however both branches were subject to OKW authority and most Waffen-SS and Heer units were prertty much used interchangably in most combat roles.