Mormons believe both passages,
(1) That Adam and Eve were commanded to multiply (Genesis 1:28).
(2) Unless these partook of the fruit of the tree of knowledge Adam and Eve could not have children (2 Nephi 2:23).
And along with other Christians, they also believe
(3) God commanded them not to partake of the tree of knowledge of good and evil (Genesis 2:17).
So really, the contradiction in Mormon belief seems to be in the commandments God gives to them: multiply and don't eat...but you can't multiply until you eat.
God is forcing them into a situation in which they cannot follow his commandments. Is God the author of sin?
Other Christians don't face this contradiction because #2 is unique to Mormonism.
AFAIK, there isn't a direct, clear-cut answer to this in canon, but here are two scripturally-supported common explanations.
Explanation 1: It was a transgression, not a sin
Mormons consider transgressions and sins differently.
A transgression is violating a law. A sin is violating a law, knowing it to be evil.
Joseph Fielding Smith, a President of the LDS Church said
I never speak of the part Eve took in this fall as a sin, nor do I accuse Adam of a sin...This was a transgression of the law, but not a sin...for it was something that Adam and Eve had to do!
Just as Mormons believe little children cannot sin (Moroni 8:8) they believe that Adam and Eve could not sin, as they did not know good from evil.
Thus, God's commandments were not a sadistic trap that forced them to sin. Instead, because they could not sin, he was offering a sin-free choice: live innocently in the garden forever without joy or pain, or leave the garden, know good and evil, and have posterity.
Eve chose the latter and Adam followed.
Explanation 2: One commandment superceded the other (for now)
Mankind (according to Mormonism and many other Christian faiths) is still under the command to "multiply and replenish the earth".
However, my adult sister is not participating in this commandment. Why? Though she could physically bear children, she is not married, and God has forbidden sexual relations outside marriage (fornication, adultery). On the one hand, she is commanded to multiply, but on the other hand, another of God's commandments prevents her (for now).
This is the situation that Adam and Eve faced. They were to multiply, but as with my sister, not necessarily immediately. Had Adam and Eve obeyed the restriction to not partake of the fruit of the knowledge of good and evil, circumstances may have changed, e.g. God lifts the restriction and commands Adam and Eve to partake of the fruit of knowledge of good and evil and they fulfill the commandment to multiply, without repercussion.
Of course, this never happened, and God knew it wouldn't, but God wasn't contradicting himself then, just as he isn't contradicting himself now.
It is worth noting that rather than viewing the Fall of Adam and Eve as an unfortunate and evil accident, one which if it had not happened, everything would be perfect, Mormons see the Fall necessary to the purpose of life on earth.
Like is an opportunity to learn to choose good over evil. This could not happen if there was not good or evil. (See 2 Nephi 2.)
And as the sacrifice of the Son of God was planned from the beginning,
And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world.
so was the Fall (necessitating the sacrifice) planned. Mormons often describe the Fall as a fall downward, but also forward.