There is no guarantee that without some form of ballast resistor or other mechanism to compensate for differences in characteristics and temperature that they will conduct equal current- if you want guarantees, connect them in series. But you can't do that with your existing supply.

You could try putting individual 50Ω 10W resistors (actual dissipation about 4.5W each), one in series with each string. That will drop an additional 15V. Even 5Ω~6.8Ω 1W resistors may make a noticeable difference if current imbalance is the issue.

There will also be differences in light output between two samples of the same LED string at the same current due to manufacturing tolerances and perhaps temperature.

There is no guarantee that without some form of ballast resistor or other mechanism to help compensate for differences in characteristics and temperature that they will conduct equal current- if you want guarantees, connect them in series. But you can't do that with your existing supply.

You could try putting individual 50Ω 10W resistors (actual dissipation about 4.5W each), one in series with each string. That will drop an additional 15V. Even 5Ω~6.8Ω 1W resistors may make a noticeable difference if current imbalance is the issue.

Keeping them the same temperature as each other (even if that temperature varies) will also help. The hotter one will tend to hog more current when they are connected in parallel, which will make it even hotter.

There will also be differences in light output between two samples of the same LED string at the same current due to manufacturing tolerances and perhaps temperature.