"Again, I tell you that if two of you on earth agree about anything you ask for, it will be done for you by my Father in heaven. For where two or three come together in my name, there am I with them." Matthew 18, 19-20.
Jesus was saying there would be a day when he wouldn't be present with his followers in body, but through the Holy Spirit in the body of believers, the sincere agreement of two people is more powerful than the superficial agreement of thousands, because Christ's Holy Spirit is with them. Two or more believers, filled with the Holy Spirit, will pray according to God's will, not their own; thus the requests will be granted.
Probably a representation of this verse.
Also, the early missionaries, Paul and Silas, Barnabas and Mark traveled in pairs witnessing the Good News of Jesus Christ.
...and as Don said possibly for safety reasons.
I would classify Mormonism as a cult and some it's original doctrines founded by Joseph Smith directly conflict with Christianity and the Bible.
Just the other day two Mormon ladies came to my door and I enjoyed speaking with them, nice people.