Is molecular weight of proteins additive?
The molecular mass in Dalton, amu or whichever unit you prefer is equivalent to the molar mass $M$ in $\mathrm{g/mol}$ that chemists prefer to use for this discussion. Both are strictly additive. The molar mass of a larger complex is the sum of the molar masses of all its components.

This is because of how the unit is defined. Focussing on molecular mass, we could see ot as mass of a molecule. The mass of a molecule is put together by adding the masses of the atoms that this molecule is made up of. If that molecule now forms larger aggregates, the mass of the resulting thing (still a single ‘thing’ albeit larger) is the sum of the masses of its constituents. And so on.

You just need to beware in case you have aggregates that are not strictly $\ce{AB}$. Consider two proteins that form an $\ce{AB2}$ complex: In this case $M(\ce{AB2}) = M(\ce{A}) + 2 \cdot M(\ce{B})$, because we have two $\ce{B}$ in the final complex.