I ran a druid/warrior and found it easier to have to caster as the main and the melee as the trailer. What that allowed me to do is have complete control over threat generation and I could start dps quicker since the warrior could charge from ranged. The problem you might run into if the melee is the main would be it needing to follow you into combat and stop moving before caster unless you took it off follow before hand which for me seemed like more of a hassle.