Alternating brakes on descents: is it really useful?
I looked at modelling this and it's surprisingly complicated.  There are quite a lot of time constants and heat flows to consider.  If I had a bunch of logging non-contact thermometers I'd consider an experiment.

In practical terms you might want to think about the failure modes if both brakes are working equally hard: If you need to slow down more but both sets of pads/rims/discs/brake fluid are already hot you have less in reserve than if one is cooling while the other is heating -- that last bit of braking might be enough to tip you over the edge. 

For a finite length of descent, getting one brake hot enough to lose significant amounts of heat early in the process can help, but heat transfer is linearly proportional to temperature difference,  which will counteract the effect of having one brake hot early. The front brake will get more airflow and so cool faster than the back brake; but if you need to stop you'll need the front brake to work well (and predictably), so running downhill with the front brake lightly on probably isn't a good plan.

For the same total heat input per brake, if the peak temperature of the braking surface is higher it will lose heat into the air faster, potentially (and this is the hard part to model) reducing the transfer to the rest of the system (tubes for rim brakes, fluid for hydraulic discs).  In addition this can only increase the heat lost to air from the working face of the pads as a hot pad touching a hot rim can't give up heat, but a hot pad close to a hot rim with air blowing though the gap can. This is more applicable to rim brakes as the airflow over the pads in a disc sysyem is tiny -- clearances are smaller and there's often a mechanism blocking the airflow. Most pads have poor thermal conductivity meaning little or no heat out the back (sintered metal disc brake pads are an exception).