You can definitely recover another Trezor with the same backed up passphrase and both units will have the same seed and key set. Basically the restored backup would be identical to the original.
I wasn't aware you could do that, but still you don't want two devices to share the same seed and key set. If one Trezor gets stolen then the other is also unsafe.
Currently I have a backup unit, but I've kept it in the box and have not used it to recover my main unit's seed value. That's because I figured that if I never use the backup, I might as well leave it in the box unopened. Maybe in the future they'll come out with a Trezor 2 and I can sell or give away my unopened backup.
As an alternative to having a backup Trezor, you could restore the passphrase to a software wallet. I think that Electrum V2 supports the same standard, but I'm not sure.
Of course you'd only want to restore to a software wallet if it was an emergency, like your Trezor was stolen and you want to move the funds out fast to another wallet under your control.
Yes, I agree having a backup Trezor is a good idea. I just wanted to make a point against what philipma1957 said, he was suggesting to buy 3 trezors but have them all be clones of each other (which is a terrible idea)
A safety deposit box is the same as a bank account. You don't control it..the bank ultimately does.
Yes, but it's not like the bank owns the contents of the box, you do. It's illegal for the bank to open the safe deposit box without you being present.