You will better work using the merge statement,
something like :
merge into table_cible c
using ( select o.col1, t.coln from table_one o
inner join table_two t
on t.col1 = o.col1 ) s
on s.col1 = c.col1
when matched and c.col2 = 1 then
update set c.coln = s.coln.
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