You'll need to use an INSTEAD OF trigger. For example:

USE Sandbox;
GO

CREATE TABLE Table1 (ID int, String varchar(10));
CREATE TABLE Table2 (ID int, String varchar(10));

INSERT INTO Table1
VALUES (1,'ABC'),
   (2,'ABC'),
   (3,'ABC'),
   (4,'ABC'),
   (5,'ABC');

INSERT INTO Table2
VALUES (6,'DEF'),
   (7,'DEF'),
   (8,'DEF'),
   (9,'DEF'),
   (10,'DEF');
GO

CREATE VIEW Table1And2 AS
  SELECT *
  FROM Table1
  UNION ALL
  SELECT *
  FROM Table2;
GO

SELECT *
FROM Table1And2;
GO

CREATE TRIGGER GoToTable2 ON Table1And2
INSTEAD OF INSERT
AS
  INSERT INTO Table2 (ID, String)
  SELECT i.ID,I.String
  FROM inserted i;
GO

INSERT INTO Table1And2
VALUES (11,'GHI');

SELECT *
FROM Table1And2;
SELECT *
FROM Table1;
SELECT *
FROM Table2;
GO

DROP VIEW Table1And2;
DROP TABLE Table2;
DROP TABLE Table1;