Extending range is not a big problem with the TC or any apple wireless router.. there is a specific function to do it.. although mixing the new AC model into the older N ones has been giving people some issues.
So you have two choices.. wireless extend.. or use ethernet or homeplug (EOP) adapters to connect them as a roaming network.. the later is much the better with ethernet at least.. homeplug has the same issues as wireless.
You cannot put the TC into the dead wireless spot. You need it half way so it gets good signal and can extend it.
Apple have loads of info but the stuff is out of date cf the new 6.3 utility.
Look up extend wireless .. it is about 10% of all posts over in airport area..
Here is one for setting up roaming on 6.3 https://discussions.apple.com/thread/5134082?tstart=0
Apple doco on extend.
The rest of the setup is same.. both will be available on both computers and you can backup to either.
It is hard to start with nothing.. read the info available and when you get to the setup yell if you have problems.
But you need to decide how you want to do it. And learn how to use the 6.3 utility.
The actual setup of extend.. is simplicity itself. If everything works right.
You just factory reset the old TC so the computer can pick it up.. whilst it is connected to the new TC by wireless or ethernet. You then get a setup wizard via the utility that will step you through joining the wireless and putting in the password.. voila. takes one min. The setup wizard will configure the old TC correctly.. and that is the best way to do it.
Thanks La Pastenague,
Assuming I use an Ethernet connection between the two time capsule, will this interfere in the new capacity or range of the new ac wireless network. I mean, does the new TC using the ac network will have a better signal and range and the old one being ag-n have a shorter range, or because they are both linked together, will they both be using the downgraded n network with slower and shorter range?
Assuming I use the new iPhone 5s or 6 which will most likely have the ac capability, and my iMac and MacBook Pro which are ag-n.
It actually doesn't matter if you wireless extend or setup roaming via ethernet link (although this is better for anything at the end of the wireless bridge)..
AC will always transmit AC.. having an older TC connected at N to it.. is no different to having any other N clients.. N clients will link at N speed.. AC clients will link at AC (assuming they are close enough). N and AC will work very happily together.. as does G and N btw.. it is just the same. Each one makes the best use it can of the connection..
The slow down if any will be because wireless is a shared media.. it cannot link and simultaneously transfer data to multiple clients.. (ethernet can). So every client gets a time slice.. AC get their slice which is faster than N clients when they get their slice.. each gets the same time.. but AC is able to do more with its slice of time..
Think of a big pie.. instead of dividing it into equal bits.. I divide by time.. each person who eats the pie gets equal time.. the person with the biggest mouth and fastest eater gets more than the small mouth and slower eater.