What do you think the advantages and disadvantages are to remarrying at age 40? Both parties have elementary-school aged children from their previous marriages and they cannot have more children any more. Both are well established in their jobs. They are committed to being together and building a future together. Do they need to get married? What are the pros and cons?
Plenty of pros/cons no matter which decision they make. Do the kids even like the potential life partner? Are they getting along?
Some pros.
They will have legal benefits to getting married like taxes & medical decisions.
Children would feel secure in allowing themselves to connect to step parent since they know that the parents are planning a committed relationship.
Also their is a "social pro" to that. Children will be able to say "step parent" instead of boyfriend/girlfriend.
Cons, financially divorce is draining if that happens.
Divorce, whether you marry or live together would put children lives in upheaval and they will feel a loss all over again, esp. if children feel a close connection with step parent/partner. One set of children would have to move out forcing a total change in their whole life.
On the flipside they are going to have to decide where they are going to live to begin their life and what schools are the children going to go to, whether they marry or live together.
How long have they been divorced? Have they allowed time for the children to "heal"? How involved are the parents with the children?
You will have the "resentment" of children there because in general, kids live with the mom's and the kids of the dad's may feel like "visitors" and can get jealous of the other kids that get their dad 24/7. Esp. if dad bonds with step child well. It is just normal to feel that sting of resentment I think.
Anyway that is a few of the standards off the top of my head. Personally, it is best if they wait until the kids are older esp. if the divorces are new or there are issues with the kids. If there are too many stressors in their life, sort those out first.