@AnnieBot : I don’t think they were claiming the workload is not manageable (no need to go after their son. Also, aerospace or mechanical is often among the more rigid engineering disciplines at most schools. I don’t know about Vanderbilt, but I think of places like Georgia Tech where it is definitely a heavy hitter. BME is at least more interdisciplinary to begin with so if you have other interests in the life sciences, often the curriculum somewhat accommodates. But places like GT tend to make engineering curricula so intense course requirement wise that double majoring becomes unlikely anyway so maybe not the best comparison). I think they just do not get why so many people do it (often flippantly- not really taking either as seriously as one would expect).
They don’t see the point from the perspective of an adult. And I agree with your third paragraph as contributing to the double major phenomenon. I think what we are saying is that doing MAJORS in all is unnecessary. Often you don’t actually need a major in a discipline to access certain types of jobs usually associated with them, but you do need a skillset, some of which are often not built by the major you choose. Engineering, IF you want to do it as a career is somewhat exceptional. However, if one is a STEM major, often it isn’t too difficult to choose another department to take extra courses such as econ. up to the intermediate level at least and then a couple of upper division (or special topics) courses of interest. That, or majors like engineering often overlap well with CS, so you can take more CS courses than required and then do personal co-curricular or extracurricular projects that further enhance those skills. I think some put too much stock in the major itself and the courses and not how to properly tailor the classes of the department. It is very natural to make assumptions that the major itself actually matters at that age. I ended up landing in chemistry/structural biology. Could it have been helpful to have biology and chemistry as a major, such, but I think dabbling strongly in both could have gotten me to the same level.
My guess is that many students like you would benefit from something like a computational/quantitative neuroscience (often this type of track involves much more engineering and physics oriented courses and research but geared toward the perspective of neuroscience) program fellowship or something. You can have the cake and eat it too without having to major in both. I had a friend very much like you but he started more from the BME perspective doing research in a more BME focused lab at Georgia Tech since his sophomore year of HS and then he went across town to do the neuroscience major (he actually joined an additional lab), but he more or less powered it up and got to the point where he developed software that was neuro focused. He didn’t have the best GPA (chemistry courses kind of dragged it and neuro is actually very stringent in comparison to many other programs), but ended up being admitted to MDPhD programs. And if he did graduate school (or even a job), he would have had many options in terms of where to and what types of programs he wanted to apply because of the skills he actually developed and things he did and lesser so the courses he took (though he definitely challenged himself. Started with ochem, took analytical and inorganic to keep pre-health in play despite having the option to do gen. chem. Took many of the more difficult biology and neuro professors/courses, and took a couple of graduate courses. Also, took much more math than needed).
Aside from STEM, keep in mind that many humanities and social science majors at elites are being clever enough with how they navigate college to do more than limit themselves to grad, law, or a future MBA. Many are making themselves ready for say, consulting jobs with that single major. It can be done. The co-curricular and extra curricular can take one far, especially if they are very meaningful and marketable.