if the backscatter intensity is no longer in the product (I might have mistaken that) with the velocity, you can use the collocate module to bring both together. Then you can combine them in the Layer Manager
It is explained here where to look and get the amount of RAM installed in your computer: How to Find How Much RAM is Installed on a Computer
If it’s really 64 GB, memory should not be a problem. An explanation on how to increase the memory heap size of SNAP is given here: S2 export into tiff - NaturalRGB or Falso color infrared RGB
If you have 654 GB you can easily enter 32G for -Xmx