Is it safe to connect two extension cords to reach 75 feet to a generator?
Yes, you can run extension cords in series.  The wires have to be sufficient size for the ampacity from end to end.  The cords and in particular the junction between the cords would have to be rated for the environment, they'd have to be weather proof if they are outdoors.  

The voltage drop would be additive, you'd calculate voltage drop from the source voltage on the first cord, then calculate the drop of that adjusted voltage on the second cord.  

There is no issue with a 10-4 cord at 30 amps for 75' - that would be less than 2% at 240V and less than 4% at 120V, which should be no problem at all.