As written the view is not updateable since SQL Server does not know which rows belong in which table.

The concept you need is known as a partitioned view. To allow the view to be updatable each table requires a check constraint on its primary key defining which range of values are allowable - for example:

create table t1 (id Int constraint t1chk check (id < 10)  primary key, other columns);

See a working example Fiddle here.

I am not convinced attempting to update a view like this is a good idea but an INSTEAD OF trigger should work. Something like:

SET ANSI_NULLS, QUOTED_IDENTIFIER ON;
GO
CREATE TRIGGER V_View_UpD
ON dbo.V_View
INSTEAD OF UPDATE
AS

SET NOCOUNT, XACT_ABORT ON;

UPDATE T
SET UserComment = I.UserComment
FROM dbo.TABLE_A T
    JOIN inserted I
        ON T.GlobalID = I.GlobalID
    JOIN deleted D
        ON T.GlobalID = D.GlobalID
WHERE COALESCE(I.UserComment, '') <> COALESCE(D.UserComment, '');

UPDATE T
SET UserComment = I.UserComment
FROM dbo.TABLE_B T
    JOIN inserted I
        ON T.GlobalID = I.GlobalID
    JOIN deleted D
    ON T.GlobalID = D.GlobalID
WHERE COALESCE(I.UserComment, '') <> COALESCE(D.UserComment, '');
GO