IF the two guardrails are electrically separated, and they should be, you're right. If they're not, and I'm sure there are people who don't think of it, they can very easily short. It doesn't have to be both wheels on one axle, as long as a wheel, anywhere in the train, is touching each rail.
I found that out the hard way on my long bridge. In that case, I had insulated the guardrails from each other, but had also spiked the ends down, and two spikes at one end were touching under the ties. Try finding THAT short!