IPhone – Restoring a new iPhone from backup fails with no error or message, why

This is most likely the carrier intentionally deactivating the old SIM.

When iPhones first came out, your current phone would normally get deactivated at the moment you purchased a new phone and told the carriers to move the service to the new phone. Now, they wait until the new phone checks in to perform the swap to account for delivery delays, backups, etc...

Your first step should be to contact the carrier and explain that you want service on both phones and arrange for which number rings to which device. They might be able to accommodate this over the phone, but if the old SIM has been deactivated in the networks, most carriers are very unwilling (or unable) to re-activate the old SIM and instead ask you to get a new SIM from them at a convenient local retail spot or by them shipping you new SIM card(s).

Apple's restore process and activation of the device doesn't affect network activation and for locked phones, you just need a SIM for the correct carrier to activate the device. iOS does not check or care that the contract is expired, not established or live for iOS activation - just that the SIM is manufactured for the correct carrier.

When you restore the backup from your old phone all content currently on the phone will be erased. There is no official or user friendly way to merge iOS backup files. The methods that do exist are very hackie and involve jailbreaking your phone first but none of them are currently compatible with iOS 6.

While there is currently no way for you to merge the two SMS files together you don't have to completely loose the messages that are already on your phone. You can use a program such as PhoneView or iExplorer to retrieve the SMS files from your new phone before you restore the backup and store them as a text file. I personaly use PhoneView, but both programs (as well as a slew of others which I'm sure other folks will by happy to point out) work equally well.

As a final note, if merging the the text messages from the two backups is the only way to go for you, there is a method that is not currently compatible with iOS 6 but likely will be at some point in the future. As I said above, it's hackie and may require a jailbreak but once it's updated to work with iOS 6 will do what you're asking for. The script is available at https://smsmerge.homedns.org. I have not used this personally and can't vouch for it's safety, security or effectiveness but it has a pretty long standing history and a decent reputation online.