Two Arc’s paired - This is what we need!

I think the sound would be way off.

Because you've flipped one Arc, the left and right channels would be opposite and the upward firing speakers on one Arc would be facing downwards.

In short an expensive solution that wouldn't sound good, in my opinion.

1. Is it even possible to join two Arc’s to give a better experience? Would they complement each other?
2. If it it would work then next step would be, is it from a sound perspective doable to flip the second one and mount beneath as it would like as ONE speaker, or is limitations due to actual mounting of speakers. 
   If 1 and 2 works then you would have a button saying “flip” 2nd Arc to adjust sound set-up.

You would need an HDMI splitter but I still think that the sound would be terrible.

No, I do not agree.  Surround sound, especially Atmos, is mapped to a room.  If the room is not set up optimally, the sound is degraded.  This would be a terrible idea.  More speakers does not mean better sound!!

What problem are you thinking ‘we’ would solve with this setup?  The only benefit you’ve mention is that you think it would be ‘cool and nice looking’.  Well, if that’s all you really hope to gain, then just but a 2nd Arc and flip it upside down under your existing Arc.  No need to set it up or even plug it in.

Agreed, I had a typo in my reply where I missed the n't off would! 🤦‍♂️🤦‍♂️

To delve into this even more:

1. Your front soundstage would be non-existent because the Right/Left of the flipped speaker would now be Left/Right, clashing with the properly mounted one.
2. Your properly mounted Atmos speakers would be firing up to reflect sound off the ceiling and down to the listener.  However, the flipped Atmos speakers would be useless, firing down at the floor to reflect back up to the ceiling.

In all honestly, that should be enough to shelve the idea. 

Note that it is not possible to add a pair of FIVE’s as external, additional L/R speakers for ARC.

But even when the speaker could be added and flipped, what would be the benefit?

The Arc is for Atmos but downfiring speakers on a soundbar seem useless.