Sharing data and members w/ two vB installs

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Hello everyone,
First and foremost, I'd like to say that I will have active lic. for both installs. I do not want the lic. to become an issue.

Second, I am a novice at best. I stumble through most tasks and can usually accomplish what I am trying. Although, in truth, it probably will include a few failed attempts and way more time than it probably should take me.

On to my task.

I have a leased vB install that is active until late sept. 09.
I want to keep it up and running on the existing domain until the lic. expires.
I have another domain that I want to utilize another vB install for which I will be buying, not leasing the lic.
I want to have these two installs share content and membership between them.
I have searched around and see where more than a few people have done this. However, no one seems to want to share how it is done.

Can someone walk me through the steps needed to accomplish this task?
I understand if this shouldn't be discussed on a public forum, If needed, we can use PM's