I concur with Peter, but would add that you can create a Site Model Snapshot. This is a copy of the model which has frozen graphic settings (e.g. existing or proposed, mesh solid, 2D contours etc). It is a snapshot of settings, not of the development of the model, so if it's set to show proposed, it will continue to update as you change the main model. The Snapshot is created on the Object Info palette with Create Site Model Snapshot. Once created, put it on a different layer. Then, one viewport can include the main model and another can show the snapshot.