Yeah, two models was our original intent with this particular project we're working on, but in reality that just doesn't really pan out. First this assumes that the client will give you the time needed to fully complete the existing model including material appearances before starting the reno work (Ha!). Even then, inevitably what happens is that once you start the reno work in the new model then you find you need to add detail or update something in the existing conditions, so now you have to update two models.
The reason the need for material phasing is so poignant for this project (and many others of ours) is that it's a huge historic cathedral, and having existing conditions renderings is just as much about having virtual record of what is there now as it is to be able to show the reno work in the context of remaining existing conditions.
I guess my faint hope in posting here is that after three years of AutoDesk focusing on the ability to model REBAR in Revit, that they might actually revisit their core users and focus on improving (and in some cases even finishing) features that need more attention like phasing, materials, stairs, roof layers, etc. etc. etc.