Check out Lassus’ two-voice motets, a couple of which are Marian.

https://cpdl.org/wiki/index.php/Orlando_di_Lasso

With accompaniment, or not?
It'd hard to find things as homorhythmic as this. With beginners, it's hard to get their heads around people singing words at different times. In some ways it's even harder than the more-than-one-note problem.
Unaccompanied: O Maria virgo pia (anon 14th c.). Pretty homophonic, plus each line repeats, so more redundancy. Chief difficulties are the triplets, and the voices crossing all the time. It's mode V with te (F major) so you'll start it on D or so like chant.
Accompanied: Perosi Ave maris stella is particularly easy, though all the things from Melodie Sacre (by Perosi and his school) are very doable: https://imslp.org/wiki/Melodie_sacre_(Perosi%2C_Lorenzo) The Ave Maris is in vol. 1
Once people get contrapuntal independence, the world is your oyster. Renaissance bicinia (Rhau's book), continuo motets, French 19th c. duets, the whole 2-equal and organ repertoire.

My largely canonic 2-part Ave Maria is appropriate.