Running different voltage motors from one battery
This is odd \$12V*1.4A&gt;8.5W\$ you provided in your description. Considering the lack of more info and to stay on the safe side, I'm going to use the max figures.

First, do these calculations:

Power dissipated in the regulator
\$P_{REG}=(24V-12V)*1.4A\$

The regulator's internal temperature.
\$T_{j}=T_{AMB}+P_{REG}*(R_{th-jc}+R_{th-hs)}\$

Where:

\$R_{th-hs}\$ is the heatsink's thermal resistance.

\$R_{th-jc}\$ is the regulator's junction to case thermal resistance.

\$T_{AMB}\$ is your the ambient temperature. The max expected temperature.

If \$T_{j}&gt;T_{j-MAX}\$ which you can find on the component's datasheet, then look for another one until you find the one that suits you. Make sure to add some margins in case your input voltage gets too high for example. Follow the manufacturer's recommended schematic to make it operate correctly.

Side note: Connect the regulator to the heatsink using a thermal paste otherwise, you would have to consider an additional thermal resistance which is not good :)