Wiring 2 ac to DC power adapters together
I'll probably get downvoted but anyway, if you attempt it...


Use the same type and model of power adapter. (Make sure each can handle at least 2 Amps)
Cut both barrel plug off the both adapters.
Connect one GND to one 6V output. 
Connect other GND back to barrel plug.
Connect other 6V (now 12V with reference to the GND you just hooked up) to the barrel plug.