I heard their voices interchanging; Swift glances with themselves exchanging.
In the second sentence, I am trying to show that the beings exchanged occasional quick glances with each other.
Surely this makes more sense:
I heard their voices interchanging; Swift glances with each other exchanging.
But "themselves" sounds more aesthetic to me. The concern here is, if "themselves" can be interpreted as them looking at each other—because a being cannot look at his/her own face without a reflection; and no reflections are present.
Top answer
Your analysis is correct. " But that hurts the meter, too.
— Enoon
Your analysis is correct.
" But that hurts the meter, too.
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Your analysis is correct. The meaning you want is better expressed by "among" if you must use "themselves": "Swift glances among themselves exchanging." But that hurts the meter, too.