Under the Missouri Compromise of 1820 Missouri was admitted as a slave state and Maine as a free state.
whether new states should be admitted as slave or free states
They were admitted as part of the "Missouri Compromise". Missouri wanted to be a state, but Missouri allowed slavery. Allowing another slave state into the Union would've tipped the delicate balance in Congress in favor of the slave states. So Maine, which was part of Massachusetts at the time, was separated off and made into a new Free State (that is, a non-slavery state) to balance against Missouri, which was allowed to finally become a state.
To avoid arguments over which Western territories could be admitted as slave-states. Any new state North of Missouri's Southern border would be free soil.
the Missouri Compromise
There was an issue over whether Missouri would become a slave or free state. It also raised the question of other new states being introduced into the United States. A compromise was made to allow Missouri to be admitted as a slave state and Kansas to be admitted as a free state.
The Missouri Compromise - a line of latitude fixed at Missouri's Southern border.
Maine apexs
The phrasing of your question is a bit odd, but I interpret it as "Did the MO compromise allow 2 slave states into the Union?" If that was what you meant, the answer is no. The MO Compromise was made to keep the number senators from free and slave states equal. When it was made, the number was equal, so every slave state had to be admitted in a pair with a free state (and the other way around)(an example is the first pair that the compromise was used for: Maine was split from Massachusetts and admitted as a free state, and Missouri as shortly after admitted as a slave state; every state admitted between 1820 and 1850, when California was admitted alone as a free state (with a pro-slavery senator)). 2 slave states were never admitted at the same time, lest the compromise be broken. The MO Compromise was replaced with another deal in 1850.
The phrasing of your question is a bit odd, but I interpret it as "Did the MO compromise allow 2 slave states into the Union?" If that was what you meant, the answer is no. The MO Compromise was made to keep the number senators from free and slave states equal. When it was made, the number was equal, so every slave state had to be admitted in a pair with a free state (and the other way around)(an example is the first pair that the compromise was used for: Maine was split from Massachusetts and admitted as a free state, and Missouri as shortly after admitted as a slave state; every state admitted between 1820 and 1850, when California was admitted alone as a free state (with a pro-slavery senator)). 2 slave states were never admitted at the same time, lest the compromise be broken. The MO Compromise was replaced with another deal in 1850.
The Missouri Compromise was not 1850 but 1820. It settled the issue of slavery in the new territories acquired from France in the Louisiana Purchase. The Compromise of 1850 was also to do with slavery/freedom in new territories, this time the ones acquired from Mexico.
The North benefited from the Missouri Compromise by maintaining a balance of power in Congress between free and slave states. Missouri was admitted as a slave state, but Maine was admitted as a free state, preserving the Senate's balance. Additionally, the compromise established the 36°30' parallel, which prohibited slavery in the northern portion of the Louisiana Territory, thereby limiting the expansion of slavery into new territories. This helped to solidify the North's stance against the spread of slavery.