If 1 Indiamen on a ivory node generates 10 tusks then:
4 Indiamen on a single ivory node generates: 40
2 Indiamen on a single ivory node generates: 20
2 Indiamen each on two ivory nodes generate: 20+20=40
I've tested this and the supply-units-per-trade-ship is a constant figure. The distribution only matters if you want to deny other trade points from other nations.