Making Backup of offsite machine to a usb drive on that same machine

Hello,

I have two raspberry PIs with external HDDs and one is in an other location. I'm making backups of the internal drives to each other PI's externel HDD. That's working so far, btrbk is running from the local PI.
Now I thought it would be good to backup the systems also to their local external HDD, but transferring the remote source to the remote target seems to be done at 'internet' speed.
Here is a short graph:


In my config I have the two internal volumes each with two targets. The targets 'local to remote' and 'remote to local' have rate_limit set to go easy on my upload/download rate.
```
volume /.btrRoot/btrSys
target /media/backup/local/system
target ssh://my.domain/media/backup/local/system
rate_limit 1m
subvolume @
snapshot_name lRoot
subvolume @home
snapshot_name lHome

volume ssh://my.domain/.btrRoot/btrSys
target /media/backup/remote/system
rate_limit 4m
target ssh://my.domain/media/backup/remote/system
subvolume @
snapshot_name rRoot
subvolume @home
snapshot_name rHome
```
I'm using btrbk version 0.31.2
Is this a problem in my config or does btrbk opens two seperate ssh sessions for send/receive?

Start with the two volume definitions and their local and SSH targets in the report, using btrbk 0.31.2 as the reproduction context. Trace the remote-source/remote-target backup path and determine whether it opens separate SSH sessions or applies rate_limit unexpectedly. Done means the behavior is explained and the configuration or documentation guidance is clear.

Written by the indexing model from the issue text.