Make a backup instead I did not understand exactly what you meant.

UPDATE (yourTable INNER JOIN twoFieldsTable
       ON yourTable.id1=twoFieldsTable.id1)
                       INNER JOIN yourTable AS b
       ON b.id2=twoFieldsTable.id2

SET yourTable.SomeField = b.SomeField

Basically, part of your table updates ... part of your table. To avoid
confusion, I used an alias to describe the part that will supply the values
in the updating process, I used the alias 'b'.  I assumed your table with
two fields is called  twoFieldsTable, and with that, it becomes almost
evident that b.id2=twoFieldsTable.id2  will be the records supplying data,
and yourTable.id1=twoFieldsTable.id1  will be the records being updated.

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Note 1 - : nothing seems to forbid duplication in your  twoFieldsTable:

so the record with id1=006  would be updated twice (keeping only the LAST
updating value it will 'see') and will behave, in this case like, in VBA:

Dim  i As integer
       i=1
       i=2

So, it could be nice to add a no-dup index on the pair of fields (ID1, ID2)
to avoid  ... surprises.

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Note 2- :  It won't handle  recursion the way you may expected.

ID1    ID2
001
002    001
003    002
004    002

It is not sure if the row ID=003 will get the initial values of 002  (ie,
before 002 get updated itself)  or the initial values of 001 (ie, after 002
get updated). Furthermore, 003  and 004 may be updated differently (ie, one
with values before 002 get updated, and one after 002 get updated).

A no dup index on ID1 and on ID2 may help to avoid such problem, but will
not care of that problem entirely.

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Hoping it may help,
Vanderghast, Access MVP