We recently started developing same scenario in Helix-
We already developed Site A with Helix and now we got requirement to create another site with name "Site B" in same Sitecore instance.
So how we are implementing this -
we are creating folder with site name under the feature folder for example in navigation we have created folder /views/Navigation/SiteB and all the view renderings we created or copied under /views/Navigation/SiteB location.
Now In controller side we created a extension method which is giving us current site. And based on current side we are calling the view in controller action method. I am sharing the code for breadcrumb how we have implemented.
public ActionResult Breadcrumb()
var items = this._navigationRepository.GetBreadcrumb();
return this.View("SiteB/Breadcrumb", items);
return this.View("Breadcrumb", items);
For "SiteA" code will work as it was working and for SiteB we did this small change wherever it's needed.
So above example is only for if we have controller renderings and if we have only view renderings then we are simply creating view rendering under the site specific folder and there is no need to worry about the code.
As per this approach we don't need to make much changes in code side.
I found only this way to handle multisite with multiple nodes in Helix.