Can I choose multiple Missions to be unlocked at once?

At the moment the is no way no bulk select multiple Missions to be unlocked by a single Mission. Each Mission will need to be connected one at a time.
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Yes you can have one Mission unlock multiple Missions. However, you will need to connect each unlocked Mission to that Mission one at a time.
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To learn how to have one Mission unlock another Mission, please refer to this article.