After some testing in 3 point 1v1 games, I have decided that Septimus mounted on Nero is an extremely fun and odd list. I love it. My games have gone differently every time so its hard to make any true conclusions yet.
The four games so far (my memory isn't the best but these are highlights):
Game 1) Septimus was pushed off almost immediately and killed shortly after. Nero went quite crazy and killed my opponents three models in just as many turns. He was put into phase 2 after killing one model I think. A bull in last man standing is scary. Especially when he is getting 2 favor for triumph and 1 from the damage tree in phase 2.
Game 2) Nero was put into phase 2 after just a single attack from my opponent. Septimus was still riding on top. *I see nothing in the rules to suggest that a rider is dismounted when their mount is put into the next phase* Nero proceeded to
two of my opponents into a near by pit.
Game 3) Septimus was forcibly dismounted by my opponent. Khepros (ignoring push resistance) pushe Septimus of of Nero and into a pit killing Septimus immediately. His remaining 3 models then made very quick work of Nero. I learned that even with titan status and ignoring the first push (for base size), you still need to worry about pits.
Bonus Game! (3 players at 3 points each)
Nero was downed very early as one of the players was just coming off of game number 1 (above) and was very upset with the bull (and convinced the other player the bull would be a problem). Septimus then got into the fighting and as the dust settled, decided to back out of the fight. My opponents seeing my single gladiator as not much of a threat decided to fight it out and then finish Septimus off last. Septimus then counter attacked (or survived the enemy attack and then activated, I forget) and pulled out the win.