If one sheep was sacrificed for the 'Aqeeqah six years ago, can another sheep be sacrificed to complete it, or is it necessary to sacrifice two new sheep?

Aqiqah is a confirmed Sunnah, not an obligation. The proof for this is the saying of the Messenger of Allah, peace and blessings be upon him: "Whoever has a child born to him and wishes to perform a sacrifice for him, let him do so. For a boy, two sheep that are similar, and for a girl, one sheep." You can slaughter a sheep now to complete what you did previously.