You already have an Electrical Consultant, why not speak to them. They are far better placed to provide an accurate answer. They already know the current setup you have, and the layout, and requirements. If you don’t trust their judgement, then go and get another consultant. The correct answer will most likely need detailed information about how the load is used at your location, so a local consultant is the person you need to ask for an answer to this question.
What do you gain by using two PSG’s? If it is redundancy and you have the money, go for it. On the other hand, what do you lose by having two PSG’s?
And 8MVA for a potential peak load of 7.8MVA seems a bit to close to me, but hey, maybe in the electrical world thats how it works, its only in the IT world that we have CPU’s using only 20% of the capacity.