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We run an enterprise deployment which is exactly what you describe and external service with clients from various organisations. For context they are all banks and security is paramount. What makes you think it's not secure?
A very high level view:
We run a portal using active directory for single sign on. Clients then open an iframe containing their respective site.
We mimic site separation by using views of the agg layer in the database, effectively creating a silo between each client. In addition we use and enterprise deployment tool to promote content, making sure the right customer gets the right data.
Happy to chat if you want to know more.
That's beyond my skill level, Russell Christopher, Glen Robinson or Jonathan MacDonald are a few guys that pop to mind to help you with that.