There have been many things like Sizzy over the years, which open the content in multiple viewports at different sizes.
Actually, the project that led to the new version of Responsive Design Mode started out as background work for offering multiple views at the same time, so it's definitely something we've thought about!
There are various bugs filed, like Preview the current page in multiple window sizes at the same time and the meta bug for Multiple Viewports v1. There are various other bugs for follow on work, like keeping the viewports synchronized and crafting an inspection mode that makes sense for this.
At the moment, it's not active project, but I hope we'll come back to it some day! There's definitely some exciting tools we could offer with a setup like this.