I haven't kept a lungfish with stingray before but my immediate concern is the lungfish slapping the stingray and the stringray trying to eat/sting the lungfish. Both are going to be staying on the bottom of the tank and the lungfish is a thick rope of pure muscle that I've been slapped by when she got spooked and it HURTS and im worried that a stingray might harass/try to eat the lungfish leading to a fight. conversely, Lungfish actually have a very strong bite and could easily take chunks out of a stingray if it ever thinks the ray is food.
South American lungfish are relatively docile but are still very strong fish that spooky kind of easily, and i've heard of stringrays relentlessly pursuing other fish they think they can eat. Hopefully someone more familiar with stingrays could add something