One way that separate accounts could theoretically be beneficial is for asset protection. 529 plans offer some protection in the case of claims by creditors and even in bankruptcy. Their protection isn’t perfect and the details vary by state. It’s possible if one spouse loses a judgment etc that the 529 plan they own will fail to be protected for some reason while the 529 plan the other spouse owns will stay protected. We are talking pretty small odds here so the chance of benefit for this reason is slim.
Another possible benefit is in the case of divorce where one spouse cashed in the 529 instead of saving it for college. Having two would at least split the money so there was half left. If that was a concern in the divorce I’m sure the account could be split in the divorce proceedings. Having two accounts does prevent the non owner spouse from liquidating the account they don’t own before a court can rule on the assets.
529 plans also, by way of being completed gifts, remove assets from the donors estate. I can’t see a way that having separate accounts owned by each spouse would be of benefit for estate planning but it’s possible. The contribution could come from either spouse and thus be out of their estate so I don’t see how separate ownership would matter.