DrRacket’s allows you to edit multiple files in a single window via tabs. The File|New Tab menu item creates a new tab to show a new file. Each tab has its own interactions window.
In the General pane of the the preferences window, a checkbox labeled Open files in separate tabs causes DrRacket to open files in new tabs in the frontmost window, rather than opening a new window for the file.
The key bindings Control-Pageup and Control-Pagedown move between tabs. On Mac OS, Command-Left-Square-Bracket and Command-Right-Square-Bracket also move between tabs.