You can only have one powerline type adapter running on your mains wiring, that will be the master. All others must be slaved off that master unit. You will run the risk of an Ethernet bridge otherwise.
If the separate circuit still originates from you consumer unit, then all the slaved units shoud work to the one master.
There is no need to connect two together, otherwise you are going to get problems.
Communityhelp (Private website) Lots of additional information and help.
You will need to be connected to your home BT Broadband connection to view it, or any links to it that I post on this forum.