2nd level to c&c cage!! :D?

OK, so i finally built a 2x5 c&c cage (waiting to get coroplast untill thursday) but once i do put in the coroplast im getting 2 female guinea pigs... and even though i havent even bought them yet im thinking of adding a 2nd level to the cage, i really want my cavy's to be happy and healthy. so should i do a 2x4 on the top??? Pictures would be nice so i could get an idea ^.^ thanks everyone!! :D