They are not doing well, but neither could afford a split at this moment, although Carlos Ghosn case is a big issue in their relationship.
The split is not realistic to happen in the next few years, but some type of restructuring where Nissan would get more control/independence is likely.
Not sure where the data came from in the top comment, but also Renault didn't have any talks with Stellantis AFAIK, and not sure such merger would be approved atm.
"In early 2019, Fiat Chrysler Automobiles (FCA) sought a merger with French automaker Renault, and reached a provisional agreement with the company. However, the behaviour of the French government during negotiations led to the abandonment of the deal; The Economist reported that "for FCA this portended future interference." Nissan also had various concerns of the impact of the proposal on its alliance with Renault."
I meant before Stellantis happened, FCA were desperate for a merger with someone and it seems that their first choice was Renault, not PSA. ( Funnily PSA's boss, and the guy running Stellantis now, was at a high level at Renault-Nissan under Ghosn, but was pushed out). Check out the sibling comment for more details
Not sure where the data came from in the top comment, but also Renault didn't have any talks with Stellantis AFAIK, and not sure such merger would be approved atm.