sorry, that was not explained very well. you have to download anything again, which is causing traffic. does not matter on GCE though.
what i meant with "direct" was using one gdrive account as source in rclone. That would download from drive1 to host RAM and immediately upload to drive 2 again, without any disk interaction.
note: only interesting for GCE, since bandwidth is far greater than disk I/O, which is limited to 120MB/s.
note2: you get 1GBit/s bandwidth per vCPU. Therefore it makes sense to add some cores (stop instance, add cores, start again). i am using 16 cores, works like a charm.