I have a feeling that was the point of the opening song though. The opening song is an octave war, it's basically 4 voices vs 1. JUNNA starts off the song, showing that she can hit low notes like the other girls, then during the second line JUNNA, Kaname's VA, and Freyja's VA harmonize but JUNNA raises her voice the loudest showing her command at hitting strong notes. Makina and Reina are just support for this song. The song itself is a battle between Freyja and Mikumo, with Kaname (and the other two to an extent) assiting Freyja. During the pre chorus Freyja and Mikumo show off what they can do individual but during the chorus JUNNA accelerates with the power of her voice. The opening song isn't really for harmonization but to show the different strength of the voices.
Ikenai Borderline and Rune ga Pikatto Hikattarra, however show how both Mikumo and Freyja can match each others voice. When JUNNA is the lead voice of the harmonization (Ikenai Borderline), you can hear Freyja's VA lowering her voice to match Mikumo's octaves showing that she can hit strong notes too. This can best be heard at 2:11 - 2:15 of the full version of Ikenai borderline and throughout if you listen keenly. When Freyja's VA is the lead voice of the harmonization (Rune ga Pikatto Hikattarra), you can hear JUNNA raising her voice to match Freyja's VA octaves showing that she can also sing high pitched as well. This can best be heard during 1:05 - 1:16 of the full version of the song. Regardless of who is leading that harmonization both girls voice compliments each other exceptionally.
For a complete harmonizing of all 5 girls voices Koi! Halation the War is the best. The full song shows their amazing ability to harmonize. As Kawamari stated, all the songs from Walkure are different, which so far has been true. Each song has different harmonizations. Bokura no Senjō shows JUNNA and Freyja's VA harmonize equally with no lead, Silent Hacker has Reina as the lead with Makina harmoizing with her, ジリティツク☆BEGINNER has Makina as the lead with Reina harmonizing with her etc.
The only person missing a lead song is Kaname, and the last song axis is most likely (hopefully) hers.