On Shabbat, one may not carry in a Reshut Harabim. Kitzur Shulchan Aruch 81:2 says that there is an opinion that a road not having at least 600,000 people passing by it daily would not be considered reshut harabbim (public domain). If it did have this, it would.
Let's say there is a public street that has 300K people walking through it. Under the street, is a large shopping mall or other public area that gets another 300K people walking through it. Can the underground foot traffic numbers be combined with the surface number such that the street level and / or the underground level would both be considered Reshut Harabim, or are these considered separate areas that cannot be combined?