It depends on various other factors as well. If the motor outputs 60RPM @24V under load, given the motor is rated for 12V - 60V and the load is kept the same, then by doubling the voltage from 24V to 48V you should definitely see an increase in torque output since it would leave the system with twice the amount of current available for the motor to consume. Now, the question is "Is your motor rated for such a high current draw?"
The information provided in the question is not enough to draw any hard conclusions to guarantee this result. Because Speed, Voltage, and Torque are interdependent.
- Increase in Voltage, increases in the motor speed
- Increase in Voltage, increases the ability to draw more current
- Increase in Current, increases the torque output
- Increase in Torque, decreases the Speed (as torque approached stall torque, speed is effectively zero)
- You also have efficiency, and power which have non-linear relationships with respect to torque
From the graph, you can get an insight into the speed vs torque curve relationship with respect to load, current, and power. So if you get your motor's datasheet you should be able to understand your motor better by inferring the relationship curves.
Also, if you look into the DC motor speed control techniques, the voltage applied across the motor determines the speed for the most part. So if you increase the motor voltage then it will also increase the speed and if the motor is rated to draw twice the current then you would also see an increase in the output torque.
Hope this answer helps point you in the right direction.