Timestamps for dependent transactions

If you simply do read waits (and not commit waits), how do you ensure that dependent transactions are given monotonically increasing commit timestamps in the presence of skews?

For example, consider that a transaction T1 changed Row 1 column 1 and T2 (which started after T1) changed Row 1 column 5. T1 and T2 are being coordinated from different machines. How do you ensure that commit_timestamp(T2) > commit_timestamp(T1) ?