What happens if we connect two signal generators in series?
Usually the signals are not floating with respect to ground, so you can't do that - you would effectively short out one of them.  

If you have something that is floating (such as the secondary of a transformer) you can certainly put it in series with some other voltage source, even one that has one side grounded. 

It's also a convenient thing to do sometimes in simulation where all voltage sources are floating- for example to put a sine wave on top of another sine wave.