Multi Client Strategy

I nned to set up a portal, which is used by multiple client, where users can have the same name.

Something like "Miller" at CorpA and "Miller" at CorpB.

They also have different role/group assignements and are diffent legal entities.

On the logon screen, they need to enter their user id and their Organisation as distinction.

What will be the best strategy? I was thinking of something like a login screen with three fiels (User, Org, pw) and connect via Javascript userid and Org, which will make the "Unique user name" in the UME.

Any other tips from other installations?