If you boost with a linked 70 (for example vet acct to new B which is linked to new C) the toon that is grouped with the 70 and is also linked to it will not get triple xp. In the example I gave C gets triple, B does not, because B is linked to A and the kill is trivial to A).

If you want to instance boost that's why people state the way they do.

If you want to have your grouped kids tag mobs outside instances and your ungrouped L70 one shot it, I believe that still works ok.