Add the LoginSchemeExpression to the service
The LoginSchemeExpression attribute contains each LoginSchemeName separated by two vertical bars (||).
The LoginSchemeExpression attribute is read from left to right; the order in which the names are placed in the attribute is the order in which they will be authenticated. It is a good idea to place the LDAP with the most users in the left-most position of the attribute.
For example, if your LoginSchemeExpression expression attribute looks like this...
<LoginSchemeExpression>DEFAULT||LOGINSCHEME_LDAP2||LOGINSCHEME_LDAP3</LoginSchemeExpression>
...then the LDAP authentication will read DEFAULT first, then LOGINSCHEME_LDAP2, LOGINSCHEME_LDAP3. In this case, DEFAULT should be the login scheme for the LDAP that has the most users.