Assessing a machine with 2 NICs

[barcajax () gmail com]

Lets say we have a machine running critical business applications connected to the enterprise network on 2 NICs. From 
an assessment/audit point of view, is it necessary to scan both NICs using assessment tools like NMap and Nessus? Will 
both scan results produce the same findings (as in same ports and services open)?
Does the OS or applications influence the detection of ports/services on different NICs on the same physical machine?