I would like to begin by saying "Your table, your rules."
However, since you're asking for advice, here is mine. There are rules for attacking specific targets within a zone on page 251. In brief, you split up the Power and Control of the spell, so there is at least one shift of both for each target. I bring this up because if you're in a zone where a zone-wide attack is taking place, you're going to get hit.
Also, in your example, you said that the two groups are "several meters" apart. That sounds like two separate zones to me. Remember, "A zone is
an abstract region of the space the conflict takes place in, loosely defined as an area in which two characters are close enough to interact directly (which is a nice way of saying “can talk to or punch each other in the face”)," page 197, Framing the Scene.
I personally would find it hard to either talk to or punch someone who is several meters away.
Another thing you could do is use mooks. If they're not all that important to the conflict, have two or three "individuals" be one stat-block. That way, One well-placed attack can take care of them all without having to worry about petty things like splash damage :p
Again, these are all my opinions. Take them with the proverbial grain of salt ^_^
-EF