My first post shows how to apply two different Convection BCs, the first is active from 0 to 1 sec in Step 1, then becomes inactive in Step 2 that ends at 2 sec. The second Convection BC is inactive in Step 1 and active in Step 2. Both Convection BCs are applied to the same surfaces. Simply change the end time of Step 2 to 100 sec and the end time of Step 1 to 50 sec and you have what you want, assuming you have some APDL code to create the temperature dependent material properties for each BC.