In my experience you cant mix channels on the 6EXA so do as forestroke said and buy a y lead....
Oh and I would keep an eye on the battery leads, As the batteries supplied are cheap and rubbish and prone to leaking,
Check for the copper in the leads turning black. I was lucky, Only spotting it when investigating a battery that only lasted 1/2 a day.
Stick to Sanyo cells. IMO