. . . and never get them mixed.

[shootingstar]

When the river first begins to rise, you can run half a dozen of the deepest of them; when it rises a foot more you can run another dozen; the next foot will add a cople of dozen, and so on; so you see you have to know your banks and marks to a dead moral certainty, and never get them mixed; for when you start through one of those cracks, there's no backing out again, as there is in the big river; you've got to go through, or stay there six months if you get caught on a falling river.

Source: Mark Twain, "Life On The Mississippi", chapter "Completing My Education"

Does "have to" in "you have to know your banks . . ." also modify the words "and never get them mixed"? If so, what wording would be correct if you add "have to" to the words "and never get them mixed" in this sentence? Then again, what do you call this practice of English grammar if you omit "have to" in the last clause "and never get them mixed" as the author possibly did?

 

[jutfrank]

Does "have to" in "you have to know your banks . . ." also modify the words "and never get them mixed"?

Yes, it governs both verb phrases.

If so, what wording would be correct if you add "have to" to the words "and never get them mixed" in this sentence?

"... and you have to never get them mixed."

Then again, what do you cal this practice of English grammar if you omit "have to" in the last clause "and never get them mixed" as the author possibly did?

Call it ellipsis—specifically, ellipsis of the auxiliary verb.