I've never used a 0 degree bag (though I've camped at 0 degrees) but I think you'll be better off buying two bags of different ratings. You can consider buying ones that would fit inside each other, but if the loft is compressed it won't "count" as much. I have heard that the montbell superstretch bags (the lightest #7 one for example) work well as liners. I've never done this though. I use quilts myself, which can be easily layered if needed. If you're not familiar with quilts, check out jacksrbetter.com for examples.. I'm not carrying grandma's quilt!

Edit: I did a search for "combining bags" on backpackinglight's FORUMS and found a few articles you may want to read.