How to use 24V 100Ah battery to run 12V motors?
If you put your motors in series, then they share current and so torque. The speeds do not have to be equal, the sum of the speeds is controlled by the 24v supply.

Whether this is OK depends how you are using your motors. 

If they are identical, and mechanically coupled, then it's quite reasonable to put them in series and run them as a 24v motor.

If you expect them to ever have different loads at different times, then use a buck converter (or one per motor) to supply them with the correct voltage.

Is each motor driving identical but independent fan loads OK? What happens if one fan becomes clogged? Then the lightly loaded one will increase speed until its torque and current matches that of the first, so may overspeed. Of course it means only one over-current limit is needed to protect both motors rather than one each.