No levels,shelves,etc or anything anywhere near that concept for a pair no matter what you may have seen!I've seen people elsewhere saying they had it work but that is just not how it works with the majority of pairs.It is never worth the risk
Along the same lines though,definitely have a single floor cage with absolutely no tubes unless you're extremely lucky and find some with four entrances.I think I've seen something like that before,but not too sure.
All the obvious things that have been stated are true of course and should be followed exactly.No exceptions!If all else fails and you can't have just one level,get another cage and just separate them or you will most definitely run into some trouble.