OK, here's my findings. It's obvious at first and was proven at play. Make your primary toon a ranged hero and your secondary toon a melee.
To control my secondary, I use a wireless external numeric pad and keep it next to my main keyboard. To reduce relying on the mouse for the secondary, I assigned "Follow", "Select Nearest Enemy", "Select Next Enemy", and "Select Player 1 (or Player 2 if the mish is for the secondary)" to 4 of the keys (non-numeric) in the numeric pad.
Before I engage a group of villains before me, I left-click (using primary's mouse) on my secondary (that way you can see what your secondary is targeting), "Select Nearest Enemy" (using secondary's numeric pad), then "Follow". Basically, my left hand doing the shooting/healing, and my right hand doing the bashing. On occasion a villain sneaks in to attack my primary. So with a "Select Player 1" - "Follow" - "Select Nearest Enemy" - bash (one of the numbered keys) I get rescued.
Also, I suggest that secondary should have flight power.
Finally, you should make sure that the secondary's camera is pointing to the same direction as your primary. Once in awhile I would make the mistake of assuming my secondary is targeting a group before me but when actually it was targeting a group behind me.