If we have a combination of two columns as primary key then can we have auto gen

I am not sure I understand your question.

You can find info about composite keys here: forum.appian.com/.../Database_Schema_Best_Practices.html

Which can be separate from having a sequence on a column (@GeneratedValue in your CDT).

The queryEntity function can read data from tables with composite keys, however the totalCount and identifiers fields of the returned data subset will be incorrect unless a separate unique column in the table is mapped to a field in the CDT that is annotated with the @Id annotation.

From the above suggestion, having a separate column with unique values would be preferred and the generated values can be used for the composite i suppose.