What do you mean by “Play Ace and Arc simultaneously”?

1. The Ace can be used as a stand-alone headset with phone or tablet (wired or over Bluetooth) or other device that can send audio via Bluetooth.
2. The Ace can receive audio from the Arc for private listening in any codec that is supported by the Arc via the 5Ghz band sent by the Arc

So what is your “use” question beyond the  aforementioned?

My wife likes to listen at a lower volume than me. I would like to listen through the Ace separately at a higher volume at the same time. 

Unfortunately, that is not possible. I doubt Sonos has plans to make that a reality as all Sonos marketing points to the use of the Ace for private listening. One use case is NOT to disturb others.

I understand Sonos’ position on this, but would it really be too hard to make it a switchable option? I doubt that I would be the only one to find it useful.

Why not submit as a feature request? Who knows Sonos may make it possible in the 2nd Generation Ace as I doubt that could be incorporated via a firmware upgrade on this release.  But what do know..

Some TV’s can transmit Bluetooth along with the HDMI-eARC output used by ARC. Another scheme might be to connect a Bluetooth transmitter to the TV’s optical output. (Not all TV’s support simultaneous HDMI and optical output.)