That depends. Twice the fill i.e 40k cc per implant would be even more extraordinary than where she as at currently, but it would hardly be worth it. 80 kilograms of saline to the chest would pretty much make her immobile and the diameter wouldn't be that much larger. Being a sphere there are deminishing returns when it comes to visual impact for every liter that is added.
Now if you mean her being twice the diameter each (getting her in the territory of some of the larger FDD girls), she'd need about 8 times the fill amount, so that would be 160k per implant or 320k cc combined. 320 kilograms of implant would make her one of the heaviest people in the history of humanity and incapable of taking care of herself or leaving the bed pretty much.
So let's not do that, OK? She's already gone to far to be considered healthy and her life is a daily struggle, poor woman.
I love the fantasy of beach ball boobs, but unless we can have them at 10% (or less) weight of ordinary implants, no thanks...
Some average women will never be able to get tho the quadruple digits, and some have proven to be able to go beyond 20k cc. It depends on multiple factors that can't always be predicted.