nuova tabella (
  Id int NOT NULL,
  a int NULL,
  b int NULL
  );
GO
ALTER TABLE dbo.t
  ADD CONSTRAINT chk_t_A$B_not_null
    CHECK (
        (
          (CASE WHEN a IS NULL THEN 0 ELSE 1 END) 
          + (CASE WHEN b IS NULL THEN 0 ELSE 1 END)
        ) >= 1
      );
    

GO
DECLARE @Id int = 1, @a int = 1, @b int = 1;
SELECT @Id Id, @a a, @b b;
INSERT INTO dbo.t VALUES ( @id, @a, @b);

SELECT @Id = 2, @a = 2, @b = NULL;
SELECT @Id Id, @a a, @b b;
INSERT INTO dbo.t VALUES ( @id, @a, @b);

SELECT @Id = 3, @a = NULL, @b = 3;
SELECT @Id Id, @a a, @b b;
INSERT INTO dbo.t VALUES ( @id, @a, @b);

GO
DECLARE @Id int = 4, @a int = NULL, @b int = NULL;
SELECT @Id Id, @a a, @b b;
INSERT INTO dbo.t VALUES ( @id, @a, @b);

GO

elimina tabella dbo.t;
--<-----------
Id          a           b
----------- ----------- -----------
1           1           1

Id          a           b
----------- ----------- -----------
2           2           NULL

Id          a           b
----------- ----------- -----------
3           NULL        3

Id          a           b
----------- ----------- -----------
4           NULL        NULL

Msg 547, Level 16, State 0, Line 37
The INSERT statement conflicted with the CHECK constraint "chk_t_A$B_not_null". The conflict occurred in database "tempdb", table "dbo.t".
The statement has been terminated.