Lines in Berryz songs have never been spread equally, though. Well, maybe in their early singles. Really early. The leads started to get clear with Jubaku, I think.
Jubaku - 4tops
Special Gen - 4tops
Nanchuu - 4tops but not very clear? (I don't remember)
Cinderella - Same with Nanchuu
Gag - 1top
Jiriri Kiteru - 3tops
Waracchaou - 4tops
Munasuka - 2tops
Very Beauty - 2tops (Maybe 2 and a half if you count Yurina

)
It still sucks if those two don't get to sing at all. The unison lines don't sounds like they have everyone singing. But it could be that they're split into 2 groups and take turns singing the unison.