Quote - Do you guys know how multi thread rendering works? Does a 2 thread render require twice as much ram as  a single thread?

Less than twice as much, but still more. Some of the things are shared between the threads (textures, irradiance cache, raytracing data), other things are separate for each thread (current bucket + all the micropolygons in it, general renderer state). When rendering in multiple threads, you may want to consider smaller bucket sizes to compensate for that.