Can eagle autoroute same signal on two layers?
No, I don't believe the Eagle autorouter can do that.  You can set a trace thickness in the net class, but I don't know of any way to have the autorouter realize that thickness by using several thinner tracks.

As a side note, just because the tracks carry high voltage doesn't mean they need to be thick.  In fact, high voltage usually implies lower current, so if these are the tracks feeding AC into a power supply that puts out a lower DC voltage, they don't need to be as fat as those DC tracks.  The issue with high voltage is distance from other tracks.  This is also set in the net class and will be obeyed by the autorouter.

The Eagle autorouter is a decent tool that can be used to good affect, but it can't do everything for you, and you absolutely have to get really familiar with it for good results.