Sally Deford has written a lot of wonderful, free music available for download at her site Sally DeFord Music: Welcome!. Most are appropriate for Sacrament Meeting performance. I know how you feel about needing to find the right piece. It shouldn't be a performance in that it's about bringing the Spirit into the meeting, not about wow-ing everyone with your talents -- as georgia2 stated -- but you also want to select something that is truly do-able so that the beauty of the music carries the message of the Spirit well. Deford's arrangement of Abide with Me for a duet or two-part choir looks like it is in your range. http://www.defordmusic.com/abidewithmetwopart.pdf It goes up to a Db, just a half-step above middle C. If you needed to, at the end of the song where the high note is held for a long time, you could drop down an octave at measures 73 and 74. That would still sound really good, and shouldn't be too hard to make the jump down from the previous measure. Good luck! Let us know how it goes.
