How do you Combine/Compare Microsoft Project documents?

I've been trying to utilize Microsoft Project to manage my time better with current ongoing projects. I was wondering if anyone knew of a way to compare 2 projects to see how their time lines overlap and how particular resources are being used. The best way I can think of to describe what I'm looking for would be: Think an overhead projector that has a projects Gantt chart on it. To see how 2 projects overlap, you lay a transparent sheet with another Projects Gantt chart on it that appears as another color. Does that make sense?

If something like that isn't available, is there a unique way to combine 2 project documents? Or should I just do the old cut and paste from one project into the other?

The first method I mentioned about comparing would be my preferred method.Combining comes second!

Please help me out! Thanks!