In QC 5.3 there are two more useful features that simplify two-way syncronisation scenarios and improve syncronization process performance-wise.
Reverse Sync Rule allows to sync back attribute values from target to source.
Merge Sync Rule has more complex logic and can be used only on update step.
It helps to solve scenario where attribute can be changed in both source and target.
It doesn't actually detect which value is the most recent (as one may think looking at the documentation), it simply checks where value was changed and changes attribute flow direction accordingly.
In case value was changed in both source and target systems, source always wins.
For multivalued attributes Merge Sync Rule provides additional setting.