Multilayered brownie help
Multi-layered brownies are certainly possible, however if you try and make it too thick it can be too dense at the bottom because the weight won't allow any sort of rise (most brownies do rise, just not much but it's still important). I make a triple chocolate brownie with a layer of salted caramel in between and it comes out really well, that's a single recipe split into halves. 

Your idea is to make two boxes and then do 4 layers, it may work just fine, if it were me I'd do it in two batches and then place the top 2 layers on after they all cool. 

As for what to put in between you'll want to choose ingredients that are markedly different from the brownie itself. Caramel contrasts as does white chocolate, but a layer of chocolate in chocolate brownie is less likely to be noticed. 

As for the amounts of oils and eggs, follow the recipe to the letter, don't mess around with it as the consistency is already optimized. If you want to experiment do it in small batches so you can measure the results before committing to a big bake.