So I guess there's no way around that, aside from opening my 'main' account beforehand, like using a different credit card, etc.?
Well, a credit card is not important. If you credit qualify T-Mobile will just open the second account for you.
I now feel a sense of urgency since I assume this trick will no longer work once Google stops selling the phones online and leaves it all up to TM, correct?
There's really been no new news about T-Mobile selling the Nexus One. T-Mobile has never been interested in that device and with the Vibrant coming out, they're busy promoting other things. One can't know for sure, but I don't think there's a lot of urgency required.
That being said, the cheapest plan is $30 per month, 500 minutes talk. Whenever you're ready, you could open that, get the SIM card, pop it into any cheap GSM phone. Then order the phone from Google opening the second line.
When your contract is up, just port your number over to the first SIM card and drop it into the Nexus.