Can my starting SS be my Sub LB2?

Plenty of times in the past I have had my backup SS as my Sub LB2, but last night I tried to put my starting SS as my Sub LB2, and instead it seemed to be inserting my backup FS at Sub LB2.

Essentially, I'd like my starting SS to play SS in non-nickel defenses, but be my sub LB2 when I play nickel. Is this possible, or what am I doing wrong?