Answer: The Missouri Compromise made it a requirement that one free state be entered for every slave state. So it was crucial to enter in pairs to keep a balance.
Explanation: Both Arkansas and Florida were territories who wished to enter the Union but needed to enter with a free state to maintain balance mandated by the Missouri Compromise. Arkansas entered with Michigan, had they not "paired" with Michigan then Florida could have and Arkansas would have had to wait for another free state to pair with before entry.