Something to refrain from doing when you need to write a sequel:This is particularly annoying when you put Large And Significant Events in the space between the sequels, but it's always a nuisance. For one thing, it will give readers no confidence in your next happy ending. Lloyd Alexander's Westmark series had this bad; each book ended with the institution of a new form of government, and with the third, well, I suspect you would think it wouldn't last, even without the knowledge that it was France and the French Revolution with the serial numbers filed off. Having your couple who were living Happily Ever After on the outs is also foolish; it's one thing to settle down to matrimony, it's another to have them break up in the gap, (and if you object that's realistic, I will direct you to this thing called Real Life where you can get your realism twenty-four hours a day, seven days a week, uncut, and far superior to anything any work of fiction can perpetrate).
Undermining the last book's happy ending.
A less objectionable method is to shift the issues around. Let someone else fall in love, or let the prequel's heroes' child have the adventures this time.
But better yet is to make it clear in the first story that its conclusion will not mean happily ever after with everything. That there are still problems, and often of the same sort as caused the conflict in the first book, that weren't resolved in it -- and to make that clear in the middle.