In your original account (not the proxy) in nautilus (the file browser for ubuntu) right click on the directory you would like to save files in or create it and then right click and select "Sharing Options"
Next make sure to check "Share this Folder" and "Allow others to create and delete files in this folder" then click "Create Share" and then ubuntu will take it from there either your folder will become shared or you will be notified via pop up that ubuntu needs to download and install some extra packages and after it has then the folder will be shared across all user accounts and now proxy via rtmpsuck will be able to save files to that folder.
I recommend after compiling rtmpdump you install it system wide with
because this way you dont have to be in the rtmpdump folder or point to it when you switch accounts to the proxy one, simply typing "rtmpsuck" when in the proxy account will work.