Just when I thought I knew how this worked. Over a week ago I linked my wife's ticket with mine. She is a no-status Skymiles member. Traveling Saturday A-B-C. Today (5 days out) I was upgraded to F on the B-C segment and am still in C+ on the A-B segment. My wife was not upgraded to F on the B-C segment so I called. They told me that in order for my wife to get the companion upgrade, I must be upgraded on both flights. The agent confirmed that our trips are linked.