Well, MacDive doesn't support multi-tank dives yet. Do you think you could run the instructions here:
http://www.mac-dive.com/forum/viewtopic.php?f=5&t=407
...using the "vyper2" test. That way I could look at your data and use it to start working on multi tank support.
The best you could do for now is just create a new tank and add it as a piece of gear, but.. it's not going to factor into your SAC rate or anything.
If you're adding dive numbers for your HelO2 dives, go to Gear and select the HelO2, then choose your starting dive and click "Renumber" and it will do it for you.