You can ask reservations to put a note in both reservations that they are part of one trip. This doesn't guarantee anything but certainly increases your odds of being protected due to any unforeseen circumstances. I other words it's best if they know you are coming. An agent at a LAN office recently told me that it is always a good idea to request the note.
OTOH, knowing SA travel as I do --- if it were me, I would arrange to arrive in Lima the day before my connecting flight. Is this a practical option for you?