Is it possible to set two column values with one being `allow_commit_timestamp`?

Yes, this is possible, and in most cases also the preferable way to do it. Splitting it into two statements and executing these using a batchUpdate call is not recommended.

The only exception would be if you intend to read from the commit timestamp column during the same transaction. Setting the LastUpdated column to PENDING_COMMIT_TIMESTAMP() will make the column unreadable for the remainder of the transaction, so in that case it could make sense to set the LastUpdated value at a later moment. Note however that it does not make sense to do so using a single batchUpdate, as you would have no chance of actually trying to read from the table between the two update statements.