I think it would be cool if the Necromancer could enter both the fortresses
The fortresses would gain unique passives when he enters
Dol Guldur: an invisible aura around it
Minas Morgul: faster recruitment for regular troops
When entering either fortress THe Necromancer gains empowered abilities, this would work as a defence measure for both fortresses. I also think it makes sense lorewise, as he would stay and defend his fortresses and then let the Nazgul lead his armies
For cinematic purposes i think when the necromancer enters Minas Morgul it would have a green beam appearing from it and Dol Guldur could have like a dark shadow somehow appearing in it.
Thanks that is all, hope any of this makes sense, i think it would add some flavour to the fortresses!