On the Smoothieboard you can indeed run multiple power and voltage inputs… For example one configuration for the items you mention is this:

Connect the 24v supply to the "Power In 1" connector and then run your heat bed off of one of the big mosfets - P2.7 or P2.5. You will want to make sure that the power jumpers are not connected in this case.

For the 12v connection you would use that to power the main "VBB IN" to power the motors and the mixed mode outputs. The hotend would be connected to the big mosfet output on P1.23.
With the above configuration you will have a second 12v output on the small mosfet P1.22 for accessories such as fans, this could be controlled via GCode, triggered by a temperature on a thermistor or configured to be always on as needed.

If you also need to control 5v accessories you could run a 5v input to "Power IN 2" and then connect those accessories to P2.4 and P2.6.
Alternatively, if the items in question are always on you could just connect them directly to the 12v / 5v supply and simplify things.