Two tape cartridges in one media set - which one will be chosen?

Hi,

Let's say I have a media set containing one tape which is used and written daily. Let's say the allocated date is 02-Dec-2013

I now add another new tape cartridge to the same media set and erase it so the allocated date is 11-Dec-2013.

On the next backup, which tape will be used? The new one of the older one?