I've done very many of these types of models before. To maintain the separate models and still tie them together, your best option is to do a model merge. However, it is critical to create your machined model without referencing any features on the cast model or things will crash. Merge your cast model into a machined part with no features and reference only datum planes and points or a coordinate system ( the best choice). This may involve craeting a skeleton of geometric datums and points that is identical in both parts but not referenced to each other in either part. then your cuts and holes can be added and any changes made in the cast part will be carried over to the machined model. Your cuts in the machined model won't fail but they may be "entirely outside the model". This is easily addressed but redefining the cut or deleting it if it is no longer necessary. You may still be able to use your current machined model but it will require more time than it may be worth. Go into insert mode and merge your cast part first. then delete all features not in the macined model and redefine all of the remaining cuts and holes. This may sound very confusing, but sometimes you just have to jump in and spend the time. The learning curve is steep but the end result will give you what you are after.
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