At our church our children's choir - four sweet voices - just did Ave Maris Stella in both Latin and English with a little low organ harmonisation for the English verses.
All the four seasonal Marian antiphons in the simple tone.
Mary the Dawn - extremely simple and best sung antiphonally - solo+tutti or small group+tutti.
O Sanctissima. I'm sure there is a two-part version somewhere, if not it can be easily harmonised as such.
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