What is the test for disqualifying an attorney from representing a client with potentially adverse interest?

In successive representation cases, courts apply the "substantial relationship" test to determine whether an attorney should be disqualified from the successive representation of clients with potentially adverse interest. (Flatt v. Superior Court (1994) 9 Cal.4th 275, 283.) "Where the requisite substantial relationship between the subjects of the prior and the current representations can be demonstrated, access to confidential information by the attorney in the course of the first representation (relevant, by definition, to the second representation) is presumed and disqualification of the attorney's representation of the second client is mandatory . . . ." (Ibid.)