Here are a couple of the steps that I take when aiming to combine two or more parts with my middle school beginners.
It's the "before" part that matters most. When I first started teaching, I moved to fast. I thought they could hear harmonies, but it just wasn't the case. I had to TEACH them to hear them.
Some premises to remember:
a) Always work small sections. Don't try to do measures 1 through 100, for example.
2) Don't work with one section longer than a few minutes at a time. To follow the steps listed below, it may take a few rehearsals.
Here is the general idea:
Before I try to combine two or more parts, I make sure I've done the following...
1) Doubled their part as they've sung it.
2) Had them sing their part with the accompaniment.
3) Had them sing their part on various vowels (oo, ah, whatever...) with and without accompaniment.
4) Had them sing it a capella so I can catch the pitch rhythm errors and fix them...you can also begin working phrasing and diction.
Once they can sing their OWN part a capella with success in some of the drills above, they are ready to hear and work with the other part, so I quietly double the other part against their part....continuing to correct rhythm or pitch errors that can occur once you begin introducing the opposite part.
Then, I'll leave the piano and walk closer to them, and I'll quietly sing the opposite part against theirs. I gradually get louder with more repetitions.
NOW, they are ready to sing together. They must have their own linear line very solidly FIRST. Then, we can help them hear and become comfortable with chordal tuning, but we cannot rush it with young beginners in middle school. We have to carefully and methodically approach 2/3 part singing so they can feel successful and get those ears tuned up!
Processes like these help awaken their ears. I make sure that all involved parts are listening as we go through the steps. It makes the rest of the learning go faster.
When we skip steps with this age group, they get frustrated, and they misbehave!