Answering my own questions

OK.. after some additional research on the web and in the forums, here is the skinny on registering Toyota Smart key

#1 For sure... you cannot simply register (i.e. add) a used Smart Key that has been previously registered on another vehicle. You need to go through an additional reset procedure explained below

#2 Not sure for "New" smart key... From what I've read, you can simply register (i.e. add) a factory fresh Smart Key w/o doing the reset procedure... I haven't done this, so I cannot personally attest to this

To register a used Smart Key that was previously registered on another vehicle.
#1 Go into TechStream
#2 Goto Smart Key
#3 Goto Smart Key Reset
#4 Click Get Seed Code (Make sure you have JAVA installed on the PC, otherwise you just see an empty box where the seed code should be).
#5 Copy that big long seed code into the clip board
#6 goto http://sigmabit.eu/calculation-of-passcode-by-seed-code-for-toyota-lexus-scion-vehicles
Enter your SEED code and VIN and pay $26 USD. They will send you the corresponding passcode via email (mine came in about 10 minutes). They are a company in Europe, so make sure you email when they're open)
#7 Enter the passcode and then the 16 minute reset process begins... there is a timer on the screen that counts down the minutes (you must keep driver door open the entire time...)
#8 After the reset is complete, go into the register option and register ALL your keys. You register them all one right after the other. Just put the first up to the smart key button... wait for beep... put the 2nd up... wait for beep... put the 2nd up... wait for beep... and so on... when done with all the keys, click registration complete.

Caution:
#1 You get a new SEED everytime you request one via the Techstream utility... So I'm fairly sure (but not positive) you must stay in the TechStream screen where you can enter the passcode while you wait fo the Sigmabit guys to send you code. If you exit out of Techstream and get back in later once you get the email, you might need to request another seed which won't work with the pass code they send. For safety sakes, send them an email 1st and make sure they're in the office... then go into techstream, get your seed, send them the order and get your passcode w/o ever exiting techstream or turning off the car.

#2 You must regsiter all keys after entering the passcode. Otherwise you'll need to start all over again with a need seed and pass code. So make sure you've got all your keys ready to go

After all that, I now have 3 keys that work (the 2 original OEMs + the new used key)

The used key was $38 on ebay, Techstream was $25, and the pass code was $26.. so ~$90 total (still better than the dealer)

 

Smart key registration update

This thread got me to try this. First I found an unregistered smart key on E...
Then after a month of fighting with TIS, the new key programmed without having to buy a new code seed. It seems if the key hasn't ever been programmed then it's easy.
Thanks to Toyotanation

 

Looks like programing 2 cars on 1 fob would a hassle. A dealer did this for us years ago, so you could carry 1 fob for both cars. Worked well, when they were near each other they would answer in unison when locking or unlocking the doors. Would like to do this for a 12 Camery and 13 RAV4. Besides not having to carry around 2 fobs it in affect gives you 2 spare fobs, one from each car.