Complex sentence: Due to both Laura's and Charles's active involvement in the anti-slavery movement, they decided to leave the Society of Friends, and join an antislavery group of Methodists called the Wesleyans.
Split into simpler sentences: Due to both Laura's and Charles's involvement in the anti-slavery movement, they decided to leave the Society of Friends. This led to their joining an antislavery group of Methodists called the Wesleyans.