A.
Yes, indeed there are such substances. Let me explain giving an actual example. UN 2988 Chlorosilanes, water-reactive, flammable, corrosive, n.o.s. has a Div. 4.3 primary risk and also possess Class 3 and Class 8 subrisks. Div. 4.3 and Class 8 need segregation. DGR 9.3.2.1.2 is saying that you can load UN2988 and UN2988 next to each other, or make an All packed in one, as long as the UN number is the same. If the UN number is different, you cannot load in proximity nor make an APIO.

There are a few other such substances. See below:

UN 2965 Boron trifluoride dimethyl ethyrate 4.3 (3, 8)
UN 3094 Corrosive liquid, water-reactive, n.o.s. 8 (4.3)
UN 1183 Ethyl dichlorosilane 4.3 (3, 8)
UN 1242 Methyl dichlorosilane 4.3 (3, 8)
UN 1285 Trichlorosilane 4.3 (3, 8)

All these substances have hazard characteristics that require segregation. As long as the UN number is the same, you need not segregate. If the UN number is different, you must segregate.