Sorry, it's not possible.  You would have to remove it from one and add to the other.

anything is possible….they just choose to make you buy more speakers instead of integrating

I could tell you about the reconfiguring and the difference in connections that make it impossible, but you seem to have it all figured out, so I'll pass.

I think it should be possible to group the Arc (+sub) and fives and then just mute the output from the Arc. I know you can’t do this right now, but all it would need is for the higher registers from the Arc to be muted - there might need to be an EQ adjustment for the Fives to avoid “over-bass”, but technically this should be OK - after all the Arc is still doing the heavy lifting/cross-over management and 5Ghz link to the sub etc, but not making any sound itself. I also have an open plan space that would benefit from this - currently I just group them all, but if the Beam was muted the stereo effect would be more pronounced.

Yes, anything is possible.  The music quality, and overall quality, from the ARC is very disappointing. The center channel is not loud enough and there is no option to adjust. Bass output is non-existent.  I’d almost rather use the Five speakers sometimes and lose the ARC all-together. while running the One’s for rear surround.  

The Fives aren’t terrible for music, but they definitely need subwoofers paired with them.