Do I have to use multiple "that"s for the multiple clauses?

Question

Which is correct between the followings?

We assume that (clause), and that (clause).

We assume that (clause), and (clause).

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My sentence is as follows:

We define a function f with respect to x, which indicates performance of the computer. We assume that the function f is a quadratic form, and that the larger the function value is, the better the performance is.

Answer 1

Repeating that makes the structure of your sentence clearer, and clarity is a virtue.

Without that ...

"and the larger the function value is, the better the performance is"

could be taken as an appended fact, not as a second assumption.