create table T1(aid int, aname varchar(5))
create table T2(bid int, bname varchar(5))

insert into T1
values(1,'a'),(3,'b'),(10,'c')

insert into T2
values(1,'aa'),(1,'aa'),(3,'b'),(10,'c'),(10,'c')


with t as
( select aid,aname,
  row_number() over(order by aid) rn from t1
)
update b
set b.bid=a.rn
from T2 b
inner join t a on b.bid=a.aid;

with t as
( select aid,aname,
  row_number() over(order by aid) rn from T1
)
update a
set a.aid=b.rn
from T1 a
inner join t b on a.aid=b.aid;

select * from T1
/*
aid         aname
----------- -----
1           a
2           b
3           c

(3 row(s) affected)
*/

select * from T2
/*
bid         bname
----------- -----
1           aa
1           aa
2           b
3           c
3           c

(5 row(s) affected)
*/