thanks @justarandomgeek and @BrumoD for your answers!
I found the hint for 'current sharing' on the Meanwell website.
The RT65-B which i use, does not have this feature.
But in the Datasheet i found that there will be headroom up to 1A on the -12V line as long as the total power consumption does not exceed 64.4W
so instead of trying to connect the PSU's parallel i am going to add another RT65B in this specific row and everything should be fine...
thanks again!