There are several things you need to think about.
1. What genre is this? Romance? Science Fiction? Fantasy? These can and do intermingle, but there is always a dominant genre.
If your dominant genre is romance (not to be confused with a love story--that has different expectations) your reader is going to expect the relationship between the hero and heroine to be the center of the book. It's the 'big thing at stake'; it's at the center of your climax. It's the last big important thing that gets resolved; the one that is left hanging until right before The End. Nothing else overshadows it, although (obviously) other subplots can and do exist.
Under this (romance) set of reader expectations, you are promising a happily ever after or happy for now ending. You are guaranteeing that your reader will get to see a hero and heroine get together by the end of the book. You are telling the reader that you will bring them a tale where love is valued and validated by the actions within its pages.
Your first novella's details seem to do this just fine. Your second story, with marriage, kids and a possible divorce (with the same couple) sets up a very different situation.
There *are* series of stories where characters are in relationships and we see them go on more than one adventure (although admittedly I hear more about will they / won't they get together or make permanent an attraction, rather then will they avoid getting divorced. But do some research or decide to ignore the trends and tell your tale if you wish.) However, these tend to put whatever other genre romance is being blended with at the forefront. A fantasy, sci-fi, mystery, etc. reader may be okay with a character's "life" outside the events of their genre happening over a series of events or adventures. The romance is a bonus, or a complimentary ingredient to the overall package, rather than being the thing at its core.
Figuring out what makes your book tick--what its heart or engine is, essentially--will make your life much easier. Then you will be in a much clearer position to decide whether more than one tale is needed, whether they should be blended together, etc.
What are your goals as the writer of this?
Who are you hoping will read it?
The first will determine the second, and your best chances of success will likely be when you can be honest with yourself in the first question and recognize the outcome that will provide in the second. Of course it is possible to shift one's vision to make things line up, and provided the end result still feels true to you that's totally fine. But making sure you know what you want both as a writer (how the story will go, what will happen to the characters, etc.) and what you want as an author (who will read this? what do you want to make your readers feel? What promise(s) are you making them?) all seem good questions to ask.
The other thing I will say to you, as someone who is working on a set of novellas, is that for me, the important rule of thumb in plotting / outlining / writing my stuff is that while each story does connect to the greater whole of my work, each story is a complete and self standing piece of fiction with a clear beginning, middle, and end. They do have different types of endings (fortunately the overall genre I'm writing is fantasy, even though romance plays a big role.) and they emphasize different sets of circumstances, as you seem to be doing.
I think the best way to put what I'm saying here is this: please don't pull a Lord of the Rings / Tolkien thing on us. (The book was split into pieces because it was way too big, but it was essentially just "cut"--each piece didn't have a completely distinct story / structure to it. I still remember how my mom reacted to the movies: "Wait. Where's the rest of it?")
I hope something in here is of help. And as always, this is just my opinion. Take what helps and toss the rest.
Good luck!
