How to copy datafile in the same ASM DG

Hi, I have one large diskgroup for data in my Exa and what I'm trying to achieve is to use transportable tablespace feature to copy tbs from one db to another. I have the tablespaces in RO, I already have metadata out, now I just need to copy datafiles...Now, every single example I found says how to go FS-ASM, ASM-FS, DG-DG but not the simplest case of them all, one db to another in the same DG. asmcmd cp doesn't work, rman backup as copy neither, what I am left with? Seems there should be an extremely simple solution to this but I can't find it.