As far as safety, two are better than one, but as Dave Colling pointed out, they are NOT a substitute for monitoring your temps.

I have personally seen heat tape discolor and suddenly get a hot spot. When I saw this I realized how unsafe bare tape was on a tub. It can crack glass, melt plastic, kill your babies, start a fire, what have you.

Now unless that hot spot forms right under one of your probes your therm just isn't going to catch it.

Of course, murphy's law states that any failure will occur when you aren't looking for one, and if a snake dies, it will be your most expensive one.

I like the freedom breeder/ARS way of putting heat tape in a channel made of aluminum, and having the tape insulated on the bottom. (Also, I think the foil tape goes to the ground lug on my strips!!!) That forms a big heatsink that really makes the temps stable (It's never off the setpoint, ever.). It also would help slow down and spread around any hotspots that may form to increase your chances of catching it in time, as well as minimizing the effect, as your probe is more likely to catch it.

It's really degrees of safety, as you're never 100% failsafe, no matter how good your system is.

Net effect: The temps are much more stable with on/off therms than one would think. Mine seriously click at 10-30 minute intervals. Of course, since I have 2 sections of tape next to each other (With different setpoints.), that may help stabilize as well.

I also didn't mean to imply that I thought that any single thermostat is safer than any other.

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