Every couple, married or not, has their own method of dealing with household finances. There are dozens of ways to approach it, from a fully joint financial operation (like my house) to a fully separate one, with many points in between. Some work and some fail, but that success or failure isn’t necessarily caused by the system. Instead, the problems that can lead to joint financial difficulties come from other places, from disruptions outside the control of the couple, to gross mismanagement of money by one or the other.
I am not a financial planner, and wouldn’t advise any particular level of commingling over another for a couple on the verge of merging their households into one. What is key, and almost uniform among couples with healthy household finances, is clarity and communication.
Clarity means that either partner must be able to explain clearly the state their finances are in to the other before they enter the joint financial relationship. This requires you to have a clear (and honest) understanding of your finances (no self-delusion, please). Even more important is to be comfortable discussing those finances with a person who will now be impacted by those finances (directly or indirectly). And you should demand that same clarity from your partner.
Once you understand the state of your finances (and your partner’s finances), open and honest communication becomes paramount. If one (or both) of you are struggling, work together to develop a plan to manage the hard times. It may involve a joint debt reduction program, filing a bankruptcy (either singly or jointly), or some other process or plan. But laying the facts on the table and coming together to forge a plan will alleviate many of the stresses associated with debt and relationships.
Like any aspect of a relationship, finances require honesty, communication and effort to be successful. Having these important conversations before you financially intertwine will save many stressful “conversations” in the future.