Identity Theft is the buzzword for 21st century and in this regard its best practice to use SSO to integrate multiple portals and unify a person to its correct usage &/or limitations.
I suggest that the two web entities cannot use separate account systems. They are functionally independent of each other. One of them is actually hosted on salesforce while the other is by tableau software.
I use my work email to access custom.tableausoftware.com and use my gmail to access the community site. To me they are two different entities and leading to identity theft over time. One is for work and the other is for personal. Tying them together is expected.
However, when I was forced to log out one when accessing the other. How about the server lets me stay logged in on both sites for same account login? It will make my life easier because I had to maintain different credentials to access them.