I think I may have misunderstood the instructions before.  Please confirm if my current understanding below is correct:

For example, if earned wage was $20K when NRA, and earned wage was $40K when RA.  The form 1040 dual-status return would have wage of $60K, with 1040NR $20K.  Therefore, the tax liability on the 1040 dual-status return would cover the whole year income. 

The 1040NR dual-status statement just shows the income earned during NRA.  The tax liability on the 1040NR does not need to be aggregated onto the 1040 since the 1040 includes income during NRA and RA.