There is no incentive for separate SaaS companies to use a shared source of data. Each system will have data for it's own needs and will manage that data as it

There is no incentive for separate SaaS companies to use a shared source of data. Each system will have data for it's own needs and will manage that data as it see's fit. The best you could do is to have a web page for the customer that shows all of the details from each app in the same place, using API's from each SaaS platform to fetch the user data from each. If you take an example of an email address, it seems like you could just synchronise it across various systems but that ignores the different purposes that each email address might have. For example, you might have a legitimate reason to separate emails from financial apps from social networks, and games.