you can choose metric as you units and decide on scale (for example a smaller scale for mm or cm instead of m)

you can't have two projects open in one window but you can have multiple instances of blender and open a different project in each one. but there are better ways:

if the model and the scene are related it makes more sense to do both in the same project by adding what's called a Scene. it's like a new project on its own, but you can easily switch between them from a dropdown menu. what's great is that you can create a new scene by copying an existing one, or create a linked scene to an existing one (linked objects share attributes so by changing one you can change all linked instances). also you can choose a scene as the background to your current scene.

I had a use case for all of this: I had a project to create product images for a catalog. I had one scene that was basically an empty studio, with a surface, background and lights. another scene to create my models, sort of like a stockpile. then separate scenes for each final image, using the studio as a background scene and copies of the models from the stockpile scene to create the image. having the studio on a separate scene helped me manipulate anything I like without worrying about touching anything that's not the product itself.