What a great looking cake! I can't tell you how much you will need but I can direct you to the following link that I find very useful when determining fondant amounts (love all the info on her site!!):
For a 6" square it says you need 2.5 lbs of fondant to work with. HTH.
Satin Ice makes a wonderful shade of red fondant and you can buy it in a two pound bucket. That would be enough for a cake that size. I have it and I love it!
Enjoy your day!
I love the cake picture! Too cute for an engagement party!!
For a 6 inch square cake that is 4 inches high, you would need 24 oz. So 48 oz of fondant to do both boxes.
For a 8 inch square cake that is 4 inches high, you would need 36 oz and then 72 oz to do both cakes.
Hope this helps!