Are you using a data adapter and a command builder to do the update?
In that case, you can change the select statement of the data adapter to be
a SELECT with just the columns from the first table, generate the command
builder and do the update. Then change the select statement to be a SELECT
with just the columns from the second table, generate the command builder,
and do the update.
That way your original query can be a join, but then using this method you
should be able to update both database tables.