What is the use case for adding two SEDs?  I understand adding two objects that may have different seds (bulge + disk is an obvious example).  But I can't think of why I would add two bare SED functions.  I tried to scan the code for an example, and the only place I found was your unit test.

And if I did add two SEDs, I guess I would have thought that the resulting wavelength range would be the union of the two original ranges.  i.e. the sum is non-zero wherever _either_ original SED is non-zero.