Okay, there are a couple things.
The easiest is to just make it eyes and die, but I know what you mean about wanting a symmetry.
The second thing you could do is say eye instead, the implication being that eyes come in pairs.
The third will allow more creativity. Use the word gem, jewel, amber (a brown gem), or whatever to describe the eye. (e.g., my girl has beautiful eyes of amber/to say they cannot kill would be serious slander.) The point being that you can replace or rearrange one word in such a way that allows for more creativity.
Also, on a side note, love poems are personal, so you should say "for which I could die..." instead of one, unless you've been instructed not to use a personal pronoun. Best of luck.
