The simplest way is to use one of these
https://www.amazon.com/Niles-Black-FG00003-Amplifier-Selector/dp/B00006HOFR
to choose which plays the front speakers.
A standard speaker selector will common ground the speaker outputs of the Marantz and AV receiver which is not good. This one won't do that.
There is another more complicated way.
If your receiver has preamp outputs (you can't use record outs since they won't change in volume) then you connect the front L/R preout to one of the inputs on the Marantz. If not then you would use a speaker to line level converter like this one connected to the front speaker terminals
https://www.crutchfield.com/p_142LOC80/Scosche-LOC80.html
Connect any two channel sources directly to the Marantz. Connect the Marantz to the BA 360 front speakers.
You will need to re-balance the speakers on the AV receiver with the Marantz volume control set to an easy to remember position since it now affects the volume of the front speakers too.
When you want stereo turn off the AV receiver.
When you want surround sound select the input on the Marantz that you connected to the AV receiver and set the volume control to on the Marantz to your reference position.
Use the AV receiver volume to control the overall volume of all channels and the subwoofer.
The advantage here is that the improved sound of the Marantz will be there on the BA 360 when you do surround sound too.