I know your not supposed to mix species but this is sort of bending the rules since they wouldnt come into contact with each other.
Simply saying one could house mixed species is not correct either. I have done it, still do it as well as many others out there. Although, as a general rule, it is recommended to beginners to keep reptiles in enclosures by themselves. There is a huge amount of species that are perfectly compatible in same enclosure set ups. You should know the rules before you mess around with keeping mixed species together though.
I have a display case now that houses an AZ Mountain King and an AZ Black rattlesnake. These species share relatively the same identical range as one another. They share the smae basic husbandry needs and are quite compatible with one another. Yet, for the average person in herpetoculture, it is something that should not be done for several reasons.
Your friends idea is one that should have had a bit of thinking put into it. Although, I will say, an imagination is a horrible thing to waste. It is nice to have set ups much like this, but an overall planning of what is going on needs to take place here. I see no reason for this said housing scheme, but others relatively close to it can work.