Yep, that’s exactly the CrashPlan feature I miss! It would be great if Duplicati could include something like a built in sftp sever, but unfortunately you are correct - with how Duplicati works fort now you have to manually set up the “destination” server at each end.
The process varies depending on the destination OS, but here are two How-To topics that might help get you started using the S3 compatible tool Mino.
I think part of the reason it’s but as simple as CrashPlan is because working through firewalls in a simple way generally requires a sever (that both clients can talk to) that gets all the right ports open so the two clients can then talk directly to each other. Since Duplicati is client only, that intermediary is missing. For now. 
Oh, and also keep in mind that Duplicati currently only initiates backups at their scheduled time. If your Duplicati says “time to back up” but you brother is offline then the backup is skipped until the next scheduled time. With CrashPlan I believe it would repeatedly check if the destination is online then start / continue a backup.
Oh, and as far a new post goes, if this discussion gets too long or too off OP topic we can shift the new posts over after the fact for you.