backup two computers- and backup my backup

Okay--- so had a MB. Had my 2TB Time Capsule backing that computer up and in addition two external hard drives.

Now, my primary computer is going to be my new iMac and will bring some files from the MB hard drive and the two externals will probably be permanently (for most part) attached to my iMac.

I went to go try to backup my iMac with the externals on. And, my TC did not recognize the externals as the same drives (IE it wants to backup the whole entire drive again....) and, in doing such, the 2TB will be full. So, now it must begin to delete old data, which I haven't allowed yet.

I know one option I have is to move all files to where I want them. Then, delete the TC and start over (as right now I have no files that I am missing). Have it backup my iMac with the externals and my MB (if I choose to include that on my backup as well--- it is turning into my children's computer now).

However, I am still facing an issue I had before (I wanted to have a backup of my backup in the instance that my house burnt down.) Many people suggest these online backup servers, which I did for a while. Problem is they don't support backing up external drives, which is where a majority of my important info is (iMovie projects, photos, etc as those get so large that they eat up my hard drive space.)

So (per one person's recommendations on these boards), what I had done was attached a 3TB hard drive to my MB (before purchasing the iMac) and had identified that hard drive as my TM location [for argument sake we will say July 1, 2012]. It backed up etc. I had my husband take that hard drive to his work. I relinked to my TC as my TM location.... and it acted like it was gonna start all over.... but I don't "think" it did. I might be wrong on that. But, later, I tried to go back to previous days [prior to July 1, 2012] and it would not go before that date that I relinked it to the TC.

This isn't going to be my longtime solution I am guessing?

Basically, I want to ideally backup my iMac and my MB (at least until I know for sure I have everything brought over from my MB) on my TC. But, right now, the TC is at 1.5TB, and it will not begin backup of my iMac bc the externals make them exceed the capacity (even though they really are already on the TC as they were attached to my MB- and are now attached to my iMac).

However, I want to also have a workflow that covers myself in the case of theft or fire. Having a backup of my iMac and MB does me no good if either of the previous tragedies happen. And, apparently the TC is not smart enough to realize what is already on there, else it would be able to go back to when I first started doing TM a while back[October 2010] (vs relinked July 2012) and it would realize it has already backed up these two external hard drives, it is just connected to a different Apple product.

Help me backup my iMac and MB (and the externals). And help me backup my TC. Please! Thanks in advance! (Oh also... my iMac is on Mountain Lion... my MB is on Lion if that matters).