So you want to keep both iPhoto and Aperture? Why keep iPhoto? Aperture has a feature where it can import an entire iPhoto library. Do that then you can trash both the iPhoto app and the iPhoto library. Aperture Libraries are better and more flexible and ou have the vault feature. If you have to choose one library or the other I'd go for the Aperture format library.
All that said, I'm pretty sure Apertur can browse into an iPhoto library. The Apple web site has a video tutorial on how to use both iPhoto and Aperture together and share libraries. Sorry I don't have a link. It's a pretty good video that directly address your question