One model or two?

[COOKE]

For 2 years I've been using SketchUp for small-scale architectural work - residential extensions and conversions - but not new builds. I'm in the process of adopting the Sketchup Workflow for Architecture, to better organise (and understand) my layers and scenes, and improve productivity.
I'm also considering working with 2 separate models instead of just one - one for the existing conditions and one for the proposals. Does anyone have any firm opinions as to whether this would simplify the design process?

[JQL]

You really SHOULD do that!

The elements that are common to both models should be made into a 3rd model that can be synced between the other 2 with context menu save as... reload.

[COOKE]

Thanks for that JQL. What exactly is the purpose of the third model?