Sounds great, good luck with the contest!
Rose has a wedding cake in the Cake Bible that is very similar to what you’re planning, it is on p. 216.
And there’s a chart that tells you how much frosting to make for any size cake, it’s on p. 513. It sounds like you’re asking how much frosting for just the filling between the layers, is that right? If so, the chart says a total of 3 cups for the two cakes you mention.
I generally find that the amounts in that chart are an absolute minimum for a thin layer of frosting with no piping, most of the time I need a little more to fill in here or there.
Hope that helps:)