Yea, that makes sense, but if I was you(and it is actually because my setup is very similar with an iMac and a Macbook Pro), I'd delete all your old songs on your home computer and then redownload the 256Kbps versions and then use Home Sharing to get them over to your other computer.
With that said, adding your second computer does not unmatch songs. Adding your 2nd computer is like adding the library from it to the library on your home computer, that is, your iTunes Match library is a combination of both computers. On my Macbook Pro, when I turned on iTunes Match, it scanned the library, but didn't really do anything else because the songs on my Macbook Pro are exactly the same ones as on my iMac which had been previously scanned and identified when I turned on iTunes Match on my iMac. That is because I have only copied stuff from my iMac using Home Sharing. Now, if I had ripped a CD to my Macbook Pro only then iTunes Match would have scanned it, determined if it could match it and added it to the cloud as a match or uploaded it to the cloud for the rest of my devices to be able to pull it down.
The key is to remember that the library in the cloud is the sum of all your computers that have music and for which you've turned on iTunes Match.