So sorry that's happened to you!That's so sad... I would be crying my eyes out.
What thermostat were you using? Herpstats are currently the hobby's top of the line thermostat with the most precise temp control and the most reliable... But being an electronic, of course things could still happen. Just a very, very low chance.
If you buy a herpstat 6 (or whichever one you need for however many snakes you have), you can technically control per tub if you make a separate plug for each heat tape for each level, or just buy multiple UTH's of the same size.