From a scientific perspective fire and lightning actually aren't really related except for the tendency for lightning to create fires. This is due to the heat produced by the lighting itself. Fire is really the rapid oxidation of a fuel, while lightning is a massive electrostatic discharge, or plasma in a more physical form (not by much).
In our non-magical world we can manipulate fire by manipulating either the fuel or the oxygen (i.e. spraying like a flamethrower or changing available oxygen levels). Lightning functions far more radically, but we have gotten quite good at directing it with grounding wires and Tesla coils.
For your purposes, you could in a way use "magic wands" as a near literal lightning rod. Thus the lightning would strike the nearest grounding point (i.e. dude you point it at). Alternatively, if understanding is all that is necessary in your universe, I might suggest the physics of electron transference and really anything relating to plasma.
I would recommend keeping the sorcerers(?) control over lightning to a limited state. If it were finely turned then it essentially becomes control of electricity and by extension the bonds between matter itself and you end up with X3 Jean Grey.
I have read a few theories on lightning weaponry by using two high powered lasers in parallel to essentially ignite the oxygen between them into plasma. You then already have the burning affects of the lasers, and then the disintegration (for lack of a better word) produced by the lighting. This is just a more real world example that might give you ideas for how to do something similar in your universe.
That said, if you want to connect fire and lighting like they do in Avatar, I would say it isn't fire or lightning bending, but rather light bending at different levels of proficiency. I imagine that might open up all sorts of other possibilities if you start relating to the entire electromagnetic spectrum (i.e. radiation, invisibility, infrared and xray vision, etc...).