Ian,
As mentioned to you quite a few times, on various threads, it is best suggested that you head over to the McGizmo section of the CPF, and check out the sticky threads on the topic.
Once you do that, and read up on the possible specs for the two stage cap, you will be able to get a better handle on what you are wanting, and how to go about the way you would wish to obtain it.
If once you do that, and you have actual specific questions, let us know, and we can offer feedback.
Hope that helps,
John