Dell TL2000 Dual Drive

So the Dell TL2000 comes with optional 1 drive or 2 drive. i have 2 drives when i backup or archive it will use 1 drive then the next tape will go into the next drive. and vice versa. how do i get backup exec to write to both drives at the same time? do i have to pre detect how much data there is, (forget the term)?

BTW i have BE 2014.1 and Server 2012 R2. 

But it didnt work in 2010 on server 2k3 R2 either.