Having the 2nd general on the battlefield is of little value per se. As noted above, they are only a high-morale (and usually lower numbers) heavy cav unit (nothing wrong with that usually, it's a useful tactical unit).

However, having two generals with your stack is VERY useful in that:

1 assassination doesn't leave you with a captain-only army

Post battle traits have somewhere to be awarded should your primary commander "throw away his life."

Cool ancillaries acquired in battle can be shunted to the 2nd leader so that the primary can acquire another -- then pass them around to all your friends.

The second guy can be sent off on errands -- build a tower, build a fort, duck across the border and find me a band of Sarmatians -- without the need to divert and slow the infantry force.

In short, the payoff for two generals is greater flexibility on the Strategic map, not on the tactical map.