order to which I should promote with these 2 OSes

note: I'm new to domains

I have 2 licenses: 2008 R2 and 2012 Server.  We use a custom piece of software in terminal services all day in my office.  It cannot run on 2012, so I have to install it on a 2008 R2 file server.

I would like to Administrate the domain using 2012 since it seems easier to use and eventually the software I use will run on 2012 anyway.

I'll have 2 boxes - the 2008 R2 file server and the 2012 box.

I want a minimum of 2 DCs so I will promote both boxes.  Someone told me there are no more primary and secondary domains (?) so maybe this doesn't matter...

Should I promote the 2012 box (using 2008 R2 functional level) first and then promote the 2008 R2 file server box as a backup controller 2nd?  Or, should I promote the 2008 R2 box and then add the 2012 DC 2nd?