the B767 has dual jacks which are independent
Except for some russian models, most aircraft have single jacks with dual (concentric) spindles and dual drive.
Some have dual nuts in which the spindle (or balls) do run, but even those can wear at the same rate and fail together.
Alaska seems to ba a one off. The stabilizer trim actuator is understood by everybody as a very critical item, and obviously receiving appropriate attention.
However, every second time can happen for the first time... Stabilizer trim failure/runaway is a valid hypothesis for such a spontaneous dive.