Cicero does this more than once.
In addition to what you found in De oratore, we have
ea ratio atque doctrina
(also in De oratore)
ratio et doctrina praescripserit
(in De natura deorum)
indicating that Cicero does see ratio et doctrina as a single entity.
This appears to be a case of hendiadys, using what is grammatically two entities for what is ...
It is as grammatical as the English sentence "By ruling".
They are both fine, but clearly elliptic; the omitted words are clear enough, so that in the context of such an exchange the response can easily be expanded to:
Regendo [[eam (= disciplinam)] sustentabo].
[I will do it (= maintain discipline)] by ruling.
When you are asked for a detail, ...