In 2005, I was flying UA SEA-SFO and NZ SFO-AKL-SYD on seperate tickets. E-ticket on UA and paper on NZ.
In SEA, I asked to be checked through to SYD and showed them the NZ ticket. Bag was checked through and was issued boarding passes for all sectors. I just had to change the passes from UA stock to NZ stock at the gate in SFO.