create table dbo.t (id int identity primary key, v int, id2 int);
go

create trigger dbo.tr_t
on dbo.t
instead of insert
as
begin
 set nocount on;

 exec sys.sp_getapplock @Resource = N'dbo.t__insert', @LockMode = N'Exclusive', @LockOwner = N'Transaction', @LockTimeout = -1;

 set identity_insert dbo.t on;

 declare @ic int = ident_current('dbo.t');

 insert into dbo.t
  (id, v, id2)
  select
   @ic + row_number() over (order by (select 1)),
   i.v,
   @ic + row_number() over (order by (select 1))
  from
   inserted i;
end;
go

insert into dbo.t (v)
values
 (10), (20), (30);

select * from dbo.t;
go

drop table dbo.t;
go