Cross Reference For SSN

Hi

There's no need to combine them.
First since you're going to want to match using first & last names, in D1 of SS1 enter

Formula:

=A1&B1

and copy down

Do the same in D1 on SS2
With both open in memory add the following formula into C1 on SS 2 and copy down.

Formula:

=IFERROR(INDEX('[SS 1.xlsx]Sheet 1'!C:C,MATCH(D1,'[SS 1.xlsx]Sheet 1'!D:D,False),1),"No Match")

Change the 'Sheet 1' reference as necessary.