It probably can kill multiple people
From the original script for Ariel:
The SOUND OF SCREAMING.
INT. HOSPITAL - SECURITY SUBSTATION - HOLDING AREA
Jayne, Simon and River react to the continued SCREAMING... what the
hell was that?
RIVER Hell is coming to breakfast.
Simon looks at River, not certain what that means, but certain he
doesn't like the sound of it.
SCREAMS come from beyond the door. First, just the one. Then more --
all in the same agony as McGinnis when he died.
INT. HOSPITAL - SECURITY SUBSTATION - RECEPTION
ON MC GINNIS
As we left him, very much dead. Then, CAMERA DRIFTS through the
reception area... FINDING another Fed. Also dead, also bleeding out of
many orifices. CAMERA CONTINUES TO DRIFT... finding another Fed. Same
thing. And another. And another. Every Fed we've seen lies dead in a
pool of blood.
CAMERA CONTINUES TO DRIFT... finding the Blue Gloved men walking
through the carnage, completely unaffected. They walk up to the
separator door, walk through
The phrase "first, just the one, and then more" suggests that multiple people were screaming at once, which is consistent with the sonic weapon affecting an area, with people nearby being affected first. Note that
They killed several Feds (at least six).
With only two Hands of Blue and one device, unless they could somehow restrain the Feds, they might find it difficult to kill them with the sonic Device one at a time, if it had a short range. This suggests that they were able to kill multiple people at once with the Device.
From the sequence of events in the episode, it would seem that the person nearest to the Device was affected first (i.e. McGinnis), and that the other Feds were affected later.
They probably didn't need to hold it near their victims for it to work; it's simply that it took effect most rapidly in that case (as evidenced by McGinnis being the first to die). As indicated here, their odd movements and unusual affect makes it quite likely they were not at all biologically typical (i.e. cyborgs or robots). An area weapon that the user is immune to makes a very effective tool.
Note that the script dialogue has some differences from the actual filmed version.