I've played a lot of Sed v Sed at this point (2v2/3v3). Several common problems that I see while laning even vs very experienced players, in a descending order of being obvious:
(1) People don't get Counter Healing at 8. That's always a huge mistake vs another Sedna.
(2) People get Pounce early (by itself a mistake), and level it above Pounce 1 (a very very very grave one). Pounce fauking sauks vs other Sedna. Why is that? A. You sacrifice some very good skills (her freaking strong auras, or Heal, or even Silence) for a so-so skill. B. It sort of begs to be used, only to spend your mana by doing negligible damage, immediately healed up by enemy Sed's Monks/Bishops. C. Strong Heal and NR >>>>> strong Pounce, that's elementary math.
(3) For some reason, people have a trouble understanding proper positioning + HW'ed creeps damage. If I lure a Sedna that has 1000 hp more than I do to my part of the flag to get dpsed by the minotaur soldiers, I oom her and push her off the flag very quickly.
(4) People stick with Monks instead of getting Bishops. Superficially, it's a sound decision when both Sednas have CH. If you look deeper, it's not. Bishops add up to your damage significantly and are much harder to kill with NR (Monks fall to NR quickly). If you are fast enough, you can do your CH dance much more efficiently. Besides, when you resummon them, you get a much stronger heal (even within CH's area).
(5) People think hp > speed. No, no, no. It's a misconception. The wise ppapanek said to me once: "You hp Sedna. You weak. You need help. Me speed Sedna. Me strong. Me don't need no help." Elegantly put, indeed! (; Having +1-1.5k hp and no speed won't save you from ganks and will make your life during CH dance a nightmare.
(6) People misuse Silence as an offensive skill. Silence is, first and foremost, a defensive skill (for example, to be used vs an enemy Rook trying to slam your whole team, in order to let your teammate tp out, or to make the enemy retreat off the flag). Its secondary use it to secure kills vs (currently) low-hp targets. Silencing in the beginning of the engagement is stupid, anyway. You just spend a lot of mana and achieve nothing.