Focus on the first notes of the strong beats coming in together when playing together. When playing separate, practice against the metronome with the metronome dinging on the beat and then keeping the rhythm of the opposing part (i.e. ding on one then tap remaining three or four). There are probably rhythm word patterns that can help when matching the beats at a slow pace. These are only guides and help calculate the pattern, but the music won't work until you can hear the flow of bpth parts (especially the primary part) without such static phrases. I have heard: When... is. he...com-....ing.back....Work on the rhythms with clapping before trying to play notes.Itr can feel like it is impossible - but then one day it will just click and you will do it with hardly a thought.