Yes you can 'link' which is essentially cross-referencing the PNRs. You can also request companion upgrade for another pax traveling on the same flight(s) as you. Even thought on separate PNRs, you can extend free checked bag to them if you check in with them at the airport, and you can have them board with you etc.

Regarding the upgrade, the key here is that from my recent experience with a non-status member travling with an elite member, agents know how to 'link' the PNRs but don't know how to request the upgrade properly... so you will have a difficult time getting their upgrade requested and it actually happening.