You can connect front speakers to speaker Front A terminal then set the corresponding speaker pattern.
Set the two other speakers to the assignable terminal (surround back speakers). A ssign surround back to SP B under SB Assign (sur back Assign) after the FH SIZE option found on page 99 of the STR-DN840manual.
Moreover, set speaker pattern to 2.0. Set speaker selector to SP A for main room, SP B for outdoor room, SP A + B for both rooms by repeatedly pressing SPEAKERS button on the front panel of the receiver (pages 35-36 of the manual).
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I have a similar problem with my setup. I have two front speakers connected to Front A, a center speaker connected to Center, two back speakers connected to Left/Right Surround and a subwoofer. I also have two outdoor speakers connected to "Surround Back/Bi-Amp/Front High/Front B". I have run through the auto setup and have my speaker profile set to 3/2.1. When I select speaker "B", I get audio from my outdoor speakers, but I am also getting the same audio from my rear speakers. Is there a way to set this so that I only get audio to my outdoor speakers using "Speaker B"? Or do I have something else going on? Any help is appreciated!