Visual Studio's "Experimental Instance" feature is intended for developing and debugging Visual Studio extensions, and maintains a separate copy of the configuration needed to run Visual Studio. Each experimental instance can have an entirely different configuration, from theme and window layout to the extensions that are loaded.
Note "Experimental instances" were previously known as "custom hives"
By default, ReSharper (and the other .net tools, dotCover, dotMemory, dotTrace, etc.) are installed as per-machine Visual Studio extensions. This means that they are available to all users on the machine, but also that they are loaded in all experimental instances.
The Platform layer that provides Visual Studio integration to ReSharper and the other .net tools, has support to move its registration from per-machine to per-user, and per-experimental instance. This allows for ReSharper (dotTrace