I'd go for the shared prong - either 2 of the smaller ones (to wear on either side of your engagment ring) or the larger one to wear close to your hand, with your ering on top.
ITA with the suggestion to check with BlueNile (or WhiteFlash, etc). I love Tiffany too, but I also love larger stones, and would go with larger stones over a brand name.
Also, I'm a fan of the half (or 3/4) eternities. Unless you're going to walk around with the palm of your hand facing up, IMO, those diamonds are just wasted! I like my carat weight to all show at once! Also, like elizat said, eternities aren't always especially comfortable between your fingers, and the ability to size them is a good thing to consider. Even if your finger size never changes, you might pass it on to your daughter/niece, etc one day!