The vast majority of better quality SV blends coming out of Oz are co-fermented. Normally the ones where Viognier is added after fermentation are simply dreadful; the Viognier content is about as subtle as a train smash, producing wine that is the anthesis of harmonious.
The one exception is Run Rig where the Viognier is added just prior to bottling to “freshen up” the wine. As good as the wine is, and it is bloody good, I did not like the 2001 as the Viognier was too apparent; however I loved the 2002 as it was a lot more subtle.
As an aside, were the grapes are co-planted, picking must be a nightmare as the two varieties would surely ripen a fair way apart.