Ok, I'm not sure I completely follow your convoluted tale here but I'll give it a shot.
AFAIK, you will NOT be able to play the same game simultaniously on two different boxes without paying for two downloads. During the license transfer process you can select the games you want available on your new box. So say you downloaded "Joe Blow Bones His Mom" and "Happy Unicorn Funtime" on the old 360 and you are gonna do the transfer, keep Unicorn Funtime on the box your daughter will be using and send Joe Blow over to your new 360 so you guys can bone his mom in slim style.
It's an inconvenient set up IMO and once you do the transfer it has to stay that way for a year before you can do it again. It's not a big deal for you cause like you said, you can recover accounts and play with your account but it sucks when you have multiple consoles and multiple users. Another thing you might want to consider now that you have more than one 360 in your house is getting a thumb drive if you plan on playing on more than one box. I know you said you have a dedicated gameroom so it may not be an issue but if you do find yourself gaming all over the place, the drive is so nice to keep your profile on. I have 3 360s in my house and between me and my kids we are always on different machines.
One more thing, if you have multiple players online you can get into open NAT issues depending on your router and whatnot.
I LOVE paying for Xbox Live! I also love that my love for it pisses off so many people.
