Confused about how the luggage allowance works for Thai VietJet and was hoping someone with experience can help shed some light.

We are traveling as a party of 2 with 2 checked bags. I'm anticipating each bag to be about 25kg.
I'm looking to book the Deluxe ticket which includes 20kg of luggage, so am trying to figure out how to pre-purchase additional baggage allowance.

When going through the online booking process, there's an option to add baggage (15kg or 20kg), but it's not clear if this "stacks" with the 20kg that comes with the Deluxe ticket. Ie. Do I get 20kg + 15/20kg if I purchase the option?
May also be a dumb question, does extra baggage option doesn't need to be a separate bag? Or can the allowance be combined to be used for a single bag, assuming each bag is <32kg?

Does Thai Vietjet weigh the entire party's luggage as a whole, and allow splitting of total allowance? For example, 2 bags at 25kg, total 50kg. Deluxe ticket 20kg*2 + 15kg extra = 55kg? Or would each traveler need to purchase extra allowance on their own.