When you remote access a Red Lion HMI, if you are remote viewing the display pages, you are controlling the actual screen. Multiple people may access it at the same time, but if you, for example, change pages, the page changes on all views of the screen including the actual hardware.
I don't know what the limit is for how many instances of the web server are allowed at once. I think the only real limit will be your network bandwidth which is pretty minimal for this sort of use and even lower if you are using Crimson 3.1 software and the enhanced web server.
I have not used any of the cell-modem based graphite modules so what I said above may not apply. My experience has been via ethernet networks some of which are wifi based vpn routers.
EDIT: I just reread your post. Do you mean two different HMIs? Or two different views of the same one?